Documentation for autocomplete

When you type various characters in markdown fields in GitLab, we have autocomplete, such as ~, !, etc. We should document these.

List of quick actions

https://docs.gitlab.com/ee/user/project/quick_actions.html

List of autocomplete characters (aside from quick actions)

~ for label
% for milestone
@ for users and groups
# for issue
! for merge request
& for epic
$ for snippets
: for emoji

Edited Nov 20, 2018 by Victor Wu