Update docs for task lists

CHANGELOG

@@ -18,6 +18,7 @@ v 7.4.0
   - Add Pushover service integration (Sullivan Senechal)
   - Add select field type for services options (Sullivan Senechal)
   - Add cross-project references to the Markdown parser (Vinnie Okada)
+  - Add task lists to issue and merge request descriptions (Vinnie Okada)
 
 v 7.3.2
   - Fix creating new file via web editor

doc/markdown/markdown.md

@@ -10,6 +10,7 @@
 * [Code and Syntax Highlighting](#code-and-syntax-highlighting)
 * [Emoji](#emoji)
 * [Special GitLab references](#special-gitlab-references)
+* [Task lists](#task-lists)
 
 **[Standard Markdown](#standard-markdown)**
 
@@ -183,6 +184,18 @@ GFM also recognizes references to commits, issues, and merge requests in other p
 - namespace/project!123 : for merge requests
 - namespace/project@1234567 : for commits
 
+## Task Lists
+
+You can add task lists to merge request and issue descriptions to keep track of to-do items.  To create a task, add an unordered list to the description in an issue or merge request, formatted like so:
+
+```no-highlight
+* [x] Completed task
+* [ ] Unfinished task
+    * [x] Nested task
+```
+
+Task lists can only be created in descriptions, not in titles or comments.  Task item state can be managed by editing the description's Markdown or by clicking the rendered checkboxes.
+
 # Standard Markdown
 
 ## Headers