Merge branch 'docs-no-milestone-issues-api' into 'master'

In docs, add `No+Milestone` for querying by milestone

See merge request gitlab-org/gitlab-ce!20784

doc/api/issues.md

@@ -37,7 +37,7 @@ GET /issues?my_reaction_emoji=star
 | ------------------- | ---------------- | ---------- | --------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `state`             | string           | no         | Return all issues or just those that are `opened` or `closed`                                                                                       |
 | `labels`            | string           | no         | Comma-separated list of label names, issues must have all labels to be returned. `No+Label` lists all issues with no labels                         |
-| `milestone`         | string           | no         | The milestone title                                                                                                                                 |
+| `milestone`         | string           | no         | The milestone title. `No+Milestone` lists all issues with no milestone                                                                                                                                |
 | `scope`             | string           | no         | Return issues for the given scope: `created_by_me`, `assigned_to_me` or `all`. Defaults to `created_by_me`<br> For versions before 11.0, use the now deprecated `created-by-me` or `assigned-to-me` scopes instead.<br> _([Introduced][ce-13004] in GitLab 9.5. [Changed to snake_case][ce-18935] in GitLab 11.0)_ |
 | `author_id`         | integer          | no         | Return issues created by the given user `id`. Combine with `scope=all` or `scope=assigned_to_me`. _([Introduced][ce-13004] in GitLab 9.5)_          |
 | `assignee_id`       | integer          | no         | Return issues assigned to the given user `id` _([Introduced][ce-13004] in GitLab 9.5)_                                                              |
@@ -151,7 +151,7 @@ GET /groups/:id/issues?my_reaction_emoji=star
 | `state`             | string           | no         | Return all issues or just those that are `opened` or `closed`                                                                 |
 | `labels`            | string           | no         | Comma-separated list of label names, issues must have all labels to be returned. `No+Label` lists all issues with no labels   |
 | `iids[]`            | Array[integer]   | no         | Return only the issues having the given `iid`                                                                                 |
-| `milestone`         | string           | no         | The milestone title                                                                                                           |
+| `milestone`         | string           | no         | The milestone title. `No+Milestone` lists all issues with no milestone                                                                                                           |
 | `scope`             | string           | no         | Return issues for the given scope: `created_by_me`, `assigned_to_me` or `all`.<br> For versions before 11.0, use the now deprecated `created-by-me` or `assigned-to-me` scopes instead.<br> _([Introduced][ce-13004] in GitLab 9.5. [Changed to snake_case][ce-18935] in GitLab 11.0)_ |
 | `author_id`         | integer          | no         | Return issues created by the given user `id` _([Introduced][ce-13004] in GitLab 9.5)_                                         |
 | `assignee_id`       | integer          | no         | Return issues assigned to the given user `id` _([Introduced][ce-13004] in GitLab 9.5)_                                        |
@@ -265,7 +265,7 @@ GET /projects/:id/issues?my_reaction_emoji=star
 | `iids[]`            | Array[integer]   | no         | Return only the milestone having the given `iid`                                                                              |
 | `state`             | string           | no         | Return all issues or just those that are `opened` or `closed`                                                                 |
 | `labels`            | string           | no         | Comma-separated list of label names, issues must have all labels to be returned. `No+Label` lists all issues with no labels   |
-| `milestone`         | string           | no         | The milestone title                                                                                                           |
+| `milestone`         | string           | no         | The milestone title. `No+Milestone` lists all issues with no milestone                                                                                                           |
 | `scope`             | string           | no         | Return issues for the given scope: `created_by_me`, `assigned_to_me` or `all`.<br> For versions before 11.0, use the now deprecated `created-by-me` or `assigned-to-me` scopes instead.<br> _([Introduced][ce-13004] in GitLab 9.5. [Changed to snake_case][ce-18935] in GitLab 11.0)_ |
 | `author_id`         | integer          | no         | Return issues created by the given user `id` _([Introduced][ce-13004] in GitLab 9.5)_                                         |
 | `assignee_id`       | integer          | no         | Return issues assigned to the given user `id` _([Introduced][ce-13004] in GitLab 9.5)_                                        |