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

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,