Merge branch 'api-mr-doc' into 'master'

Add iid to MR API response

## What does this MR do?

Adds `iid` parameter to the MR API docs.

## Why was this MR needed?

It is missing.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19578

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)

See merge request !5468
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

@@ -6,6 +6,7 @@ v 8.10.2 (unreleased)
   - Fix backup restore. !5459
   - Use project ID in repository cache to prevent stale data from persisting across projects. !5460
   - Fix issue with autocomplete search not working with enter key. !5466
+  - Add iid to MR API response. !5468
   - Don't show comment button in gutter of diffs on MR discussion tab
 
 v 8.10.1

doc/api/merge_requests.md

@@ -276,6 +276,7 @@ Parameters:
 ```json
 {
   "id": 1,
+  "iid": 1,
   "target_branch": "master",
   "source_branch": "test1",
   "project_id": 3,
@@ -350,6 +351,7 @@ Parameters:
 ```json
 {
   "id": 1,
+  "iid": 1,
   "target_branch": "master",
   "project_id": 3,
   "title": "test1",
@@ -449,6 +451,7 @@ Parameters:
 ```json
 {
   "id": 1,
+  "iid": 1,
   "target_branch": "master",
   "source_branch": "test1",
   "project_id": 3,
@@ -517,6 +520,7 @@ Parameters:
 ```json
 {
   "id": 1,
+  "iid": 1,
   "target_branch": "master",
   "source_branch": "test1",
   "project_id": 3,