Add documentation for blocking MRs

This feature needs to be documented, so here is the documentation

doc/user/project/merge_requests/blocking_merge_requests.md

+---
+type: reference, concepts
+---
+
+# Blocking merge requests **(PREMIUM)**
+
+> Introduced in GitLab Premium 12.2
+
+Blocking merge requests allow dependencies between MRs to be expressed. If a
+merge request is blocked by another MR, it cannot be merged until that blocking
+MR is itself merged.
+
+NOTE: **Note:**
+Blocking merge requests are a **PREMIUM** feature, but this restriction is only
+enforced for the blocked merge request. A merge request in a **CORE** or
+**STARTER** project can block a **PREMIUM** merge request, but not vice-versa.
+
+## Use cases
+
+* Ensure changes to a library are merged before changes to a project that
+  imports the library
+* Prevent a documentation-only merge request from being merged before the MR
+  implementing the feature to be documented
+* Require an MR updating a permissions matrix to be merged before merging an
+  MR from someone who hasn't yet been granted permissions
+
+It is common for a single logical change to span several merge requests. These
+MRs may all be in a single project, or they may be spread out across multiple
+projects, and the order in which they are merged can be significant.
+
+For example, given a project `mycorp/awesome-project` that imports a library
+at `myfriend/awesome-lib`, adding a feature in `awesome-project` may **also**
+require changes to `awesome-lib`, and so necessitate two merge requests. Merging
+the `awesome-project` MR before the `awesome-lib` one would break the `master`
+branch.
+
+The `awesome-project` MR could be [marked as WIP](work_in_progress_merge_requests.md),
+and the reason for the WIP stated included in the comments. However, this
+requires the state of the `awesome-lib` MR to be manually tracked, and doesn't
+scale well if the `awesome-project` MR depends on changes to **several** other
+projects.
+
+By marking the `awesome-project` MR as blocked on the `awesome-lib` MR instead,
+the status of the dependency is automatically tracked by GitLab, and the WIP
+state can be used to communicate the readiness of the code in each individual
+MR instead.
+
+## Configuration
+
+To continue the above example, you can configure a block when creating the
+new MR in `awesome-project` (or by editing it, if it already exists). The block
+needs to be configured on the MR that will be **blocked**, rather than on the
+**blocking** MR. There is a "Blocking merge requests" section in the form:
+
+![Blocking merge requests form control](img/edit_blocking_merge_requests.png)
+
+Anyone who can edit a merge request can change the list of blocking merge
+requests.
+
+New blocks can be added by reference, by URL, or by using autcompletion. To
+remove a block, press the "X" by its reference.
+
+As blocks can be specified across projects, it's possible that someone else has
+added a block for a merge request in a project you don't have access to. These
+are shown as a simple count:
+
+![Blocking merge requests form control with inaccessible MRs](img/edit_blocking_merge_requests_inaccessible.png)
+
+If necessary, you can remove all the blocks like this by pressing the "X", just
+as you would for a single, visible block.
+
+Once you're finished, press the "Save changes" button to submit the request, or
+"Cancel" to return without making any changes.
+
+The list of configured blocks, and the status of each one, is shown in the merge
+request widget:
+
+![Blocking merge requests in merge request widget](img/show_blocking_merge_requests_in_mr_widget.png)
+
+Until all blocking merge requests have, themselves, been merged, the "Merge"
+button will be disabled. In particular, note that **closed** merge requests
+still block their dependents - it is impossible to automatically determine if
+merge requests that were blocked by that MR when it was open, are still blocked
+when it is closed.
+
+If a merge request has been closed **and** the block is no longer relevant, it
+must be removed as a blocking MR, following the instructions above, before
+merge.
+
+## Limitations
+
+* API support: [gitlab-ee#12551](https://gitlab.com/gitlab-org/gitlab-ee/issues/12551)
+* Blocking relationships are not preserved across project export/import: [gitlab-ee#12549](https://gitlab.com/gitlab-org/gitlab-ee/issues/12549)
+* Complex merge order dependencies are not supported: [gitlab-ee#11393](https://gitlab.com/gitlab-org/gitlab-ee/issues/11393)
+
+The last item merits a little more explanation. Blocking merge requests can be
+described as a graph of dependencies. The simplest possible graph has one
+merge request blocking another:
+
+```mermaid
+graph LR;
+    myfriend/awesome-lib!10-->mycorp/awesome-project!100;
+```
+
+A more complex (and still supported) graph might have several MRs blocking
+another from being merged:
+
+```mermaid
+graph LR;
+    myfriend/awesome-lib!10-->mycorp/awesome-project!100;
+    herfriend/another-lib!1-->mycorp/awesome-project!100;
+```
+
+We also support one MR blocking several others from being merged:
+
+```mermaid
+graph LR;
+    herfriend/another-lib!1-->myfriend/awesome-lib!10;
+    herfriend/another-lib!1-->mycorp/awesome-project!100;
+```
+
+What is **not** supported is a "deep", or "nested" graph of dependencies, e.g.:
+
+```mermaid
+graph LR;
+    herfriend/another-lib!1-->myfriend/awesome-lib!10;
+    myfriend/awesome-lib!10-->mycorp/awesome-project!100;
+```
+
+In this example, `myfriend/awesome-lib!10` would be blocked from being merged by
+`herfriend/another-lib!1`, and would also block `mycorp/awesome-project!100`
+from being merged. This is **not** yet supported.
+

doc/user/project/merge_requests/index.md

@@ -47,6 +47,7 @@ With **[GitLab Enterprise Edition][ee]**, you can also:
 - Analyze your dependencies for vulnerabilities with [Dependency Scanning](../../application_security/dependency_scanning/index.md) **(ULTIMATE)**
 - Analyze your Docker images for vulnerabilities with [Container Scanning](../../application_security/container_scanning/index.md) **(ULTIMATE)**
 - Determine the performance impact of changes with [Browser Performance Testing](#browser-performance-testing-premium) **(PREMIUM)**
+- Specify merge order dependencies with [Blocking Merge Requests](#blocking-merge-requests-premium) **(PREMIUM)**
 
 ## Use cases
 
@@ -412,6 +413,21 @@ GitLab runs the [Sitespeed.io container][sitespeed-container] and displays the d
 
 [Read more about Browser Performance Testing.](browser_performance_testing.md)
 
+## Blocking Merge Requests **(PREMIUM)**
+
+> Introduced in [GitLab Premium][products] 12.2.
+
+A single logical change may be split across several merge requests, and perhaps
+even across several projects. When this happens, the order in which MRs are
+merged is important.
+
+GitLab allows you to specify that a merge request is blocked by other MRs. With
+this relationship in place, the merge request cannot be merged until all of its
+blockers have also been merged, helping to maintain the consistency of a single
+logical change.
+
+[Read more about Blocking Merge Requests.](blocking_merge_requests.md)
+
 ## Security reports **(ULTIMATE)**
 
 GitLab can scan and report any vulnerabilities found in your project.