Add documentation for releases issue summary

This commit updates the documentation to include information about the
new "release issue summary" feature. This change updates the
documentation text as well as several images in the page.

doc/user/project/releases/index.md

@@ -83,10 +83,11 @@ Releases can optionally be associated with one or more
 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.
+Releases display this association with the **Milestone** indicator in the top
+section of the Release block on the **Project overview > Releases** page, along
+with some statistics about the issues in the milestone(s).
 
-![A Release with one associated milestone](img/release_with_milestone_v12_5.png)
+![A Release with one associated milestone](img/release_with_milestone_v12_9.png)
 
 Below is an example of milestones with no Releases, one Release, and two
 Releases, respectively.
@@ -104,7 +105,7 @@ associated with a large number of Releases.
 Navigate to **Project > Releases** in order to see the list of releases for a given
 project.
 
-![Releases list](img/releases.png)
+![Releases list](img/releases_v12_9.png)
 
 ### Number of Releases