Documented autocomplete characters

- Added a table listing all autocomplete
  characters.
- Added a link to the autocomplete characters
  docs, from the Projects feature list.
- Added an example of how autocomplete would behave
  for `Users and groups`, including two screenshots.

doc/user/project/autocomplete_characters.md

+# Autocomplete characters
+
+The autocomplete characters provide a quick way of entering field values into
+Markdown fields. When you start typing a word in a Markdown field with one of
+the following characters, GitLab progressively autocompletes against a set of
+matching values. The string matching is not case sensitive.
+
+| Character | Autocompletes |
+| :-------- | :------------ |
+| `~`       | Labels |
+| `%`       | Milestones |
+| `@`       | Users and groups |
+| `#`       | Issues |
+| `!`       | Merge requests |
+| `&`       | Epics |
+| `$`       | Snippets |
+| `:`       | Emoji |
+| `/`       | Quick Actions |
+
+Up to 5 of the most relevant matches are displayed in a popup list. When you
+select an item from the list, the value is entered in the field. The more
+characters you enter, the more precise the matches are.
+
+Autocomplete characters are useful when combined with [Quick Actions](quick_actions.md).
+
+## Example
+
+Assume your GitLab instance includes the following users:
+
+| Username        | Name |
+| :-------------- | :--- |
+| alessandra      | Rosy Grant |
+| lawrence.white  | Kelsey Kerluke |
+| leanna          | Rosemarie Rogahn |
+| logan_gutkowski | Lee Wuckert |
+| shelba          | Josefine Haley |
+
+In an Issue comment, entering `@l` results in the following popup list
+appearing. Note that user `shelba` is not included, because the list includes
+only the 5 users most relevant to the Issue.
+
+![Popup list which includes users whose username or name contains the letter `l`](img/autocomplete_characters_example1_v12_0.png)
+
+If you continue to type, `@le`, the popup list changes to the following. The
+popup now only includes users where `le` appears in their username, or a word in
+their name.
+
+![Popup list which includes users whose username or name contains the string `le`](img/autocomplete_characters_example2_v12_0.png)

doc/user/project/index.md

@@ -52,6 +52,9 @@ When you create a project in GitLab, you'll have access to a large number of
   templates for issue and merge request description fields for your project
 - [Slash commands (quick actions)](quick_actions.md): Textual shortcuts for
   common actions on issues or merge requests
+- [Autocomplete characters](autocomplete_characters.md): Autocomplete
+  references to users, groups, issues, merge requests, and other GitLab
+  elements.
 - [Web IDE](web_ide/index.md)
 
 **GitLab CI/CD:**