Document Milestone <> Release feature

Adds documentation about our new Milestone <> Release feature.

doc/user/project/milestones/index.md

@@ -12,23 +12,21 @@ Milestones allow you to organize issues and merge requests into a cohesive group
 
 ## Milestones as Agile sprints
 
-Milestones can be used as Agile sprints.
-Set the milestone start date and due date to represent
-the start and end of your Agile sprint.
-Set the milestone title to the name of your Agile sprint,
-such as `November 2018 sprint`.
-Add an issue to your Agile sprint by associating
-the milestone to the issue.
+Milestones can be used as Agile sprints so that you can track all issues and merge requests related to a particular sprint. To do so:
+
+1. Set the milestone start date and due date to represent the start and end of your Agile sprint.
+1. Set the milestone title to the name of your Agile sprint, such as `November 2018 sprint`.
+1. Add an issue to your Agile sprint by associating the desired milestone from the issue's right-hand sidebar.
 
 ## Milestones as releases
 
-Milestones can be used as releases.
-Set the milestone due date to represent the release date of your release.
-(And leave the milestone start date blank.)
-Set the milestone title to the version of your release,
-such as `Version 9.4`.
-Add an issue to your release by associating
-the milestone to the issue.
+Similarily, milestones can be used as releases. To do so:
+
+1. Set the milestone due date to represent the release date of your release and leave the milestone start date blank.
+1. Set the milestone title to the version of your release, such as `Version 9.4`.
+1. Add an issue to your release by associating the desired milestone from the issue's right-hand sidebar.
+
+Additionally, you can integrate milestones with GitLab's [Releases feature](../releases/index.md#releases-associated-with-milestones).
 
 ## Project milestones and group milestones
 

doc/user/project/releases/index.md

@@ -58,6 +58,31 @@ links from your GitLab instance.
 NOTE: **NOTE**
 You can manipulate links of each release entry with [Release Links API](../../../api/releases/links.md)
 
+#### Releases associated with milestones
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/29020) in GitLab 12.5.
+
+Releases can optionally be associated with one or more
+[project milestones](../milestones/index.md#project-milestones-and-group-milestones)
+by including a `milestones` array in your requests to the
+[Releases API](../../../api/releases/index.md#create-a-release).
+
+Releases display this association with the **Milestone** indicator near
+the top of the Release block on the **Project overview > Releases** page.
+
+![A Release with one associated milestone](img/release_with_milestone_v12_5.png)
+
+Below is an example of milestones with no Releases, one Release, and two
+Releases, respectively.
+
+![Milestones with and without Release associations](img/milestone_list_with_releases_v12_5.png)
+
+This relationship is also visible in the **Releases** section of the sidebar
+when viewing a specific milestone. Below is an example of a milestone
+associated with a large number of Releases.
+
+![Milestone with lots of associated Releases](img/milestone_with_releases_v12_5.png)
+
 ## Releases list
 
 Navigate to **Project > Releases** in order to see the list of releases for a given