Link to registry limitations

Co-authored-by: Kenneth Chu <ksiuhengchu@gitlab.com>

doc/api/projects.md

@@ -2609,6 +2609,9 @@ DELETE /projects/:id/push_rule
 
 > Introduced in GitLab 11.1.
 
+See the [Project documentation](../user/project/settings/index.md#transferring-an-existing-project-into-another-namespace)
+for prerequisites to transfer a project.
+
 ```plaintext
 PUT /projects/:id/transfer
 ```

doc/user/packages/container_registry/index.md

@@ -489,11 +489,13 @@ Container Registry.
 ## Limitations
 
 - Moving or renaming existing Container Registry repositories is not supported
-once you have pushed images, because the images are stored in a path that matches
-the repository path. To move or rename a repository with a
-Container Registry, you must delete all existing images.
+  once you have pushed images, because the images are stored in a path that matches
+  the repository path. To move or rename a repository with a
+  Container Registry, you must delete all existing images.
+  Community suggestions to work around this limitation have been shared in
+  [issue 18383](https://gitlab.com/gitlab-org/gitlab/-/issues/18383#possible-workaround).
 - Prior to GitLab 12.10, any tags that use the same image ID as the `latest` tag
-are not deleted by the cleanup policy.
+  are not deleted by the cleanup policy.
 
 ## Disable the Container Registry for a project
 

doc/user/packages/npm_registry/index.md

@@ -255,7 +255,8 @@ This regex allows almost all of the characters that npm allows, with a few excep
 
 The regex also allows for capital letters, while npm does not.
 
-WARNING:
+## Limitations
+
 When you update the path of a user or group, or transfer a subgroup or project,
 you must remove any npm packages first. You cannot update the root namespace
 of a project with npm packages. Make sure you update your `.npmrc` files to follow

doc/user/project/settings/index.md

@@ -404,11 +404,17 @@ NOTE:
 Only project owners and administrators have the [permissions](../../permissions.md#project-members-permissions)
 to transfer a project.
 
-You can transfer an existing project into a [group](../../group/index.md) if:
+You can transfer an existing project into a [group](../../group/index.md).
 
-- You have at least **Maintainer** [role](../../permissions.md#project-members-permissions) in that group.
-- You're at least an **Owner** of the project to be transferred.
+Prerequisites:
+
+- You must have at least **Maintainer** [role](../../permissions.md#project-members-permissions) in that group.
+- You must be **Owner** of that project.
 - The group to which the project is being transferred to must allow creation of new projects.
+- The project must not contain any [container images](../../packages/container_registry/index.md#limitations).
+  - If you transfer a project to a different root namespace,
+    the project must not contain any
+    [NPM packages](../../packages/npm_registry/index.md#limitations).
 
 To transfer a project:
 
@@ -423,8 +429,8 @@ read what happens with the
 [redirects from the old project to the new one](../repository/index.md#what-happens-when-a-repository-path-changes).
 
 NOTE:
-GitLab administrators can use the administration interface to move any project to any
-namespace if needed.
+GitLab administrators can use the [administration interface](../../admin_area/index.md#administering-projects)
+to move any project to any namespace if needed.
 
 ##### Transferring a GitLab.com project to a different subscription tier