Mention issue closing in workflow section

Issue closing is only mentioned in the administrator docs.

We should also mention this in the user section and note that since the body of an MR gets to be in the commit message, mentioning the closing patterns in an MR description will work as well.

Milestone changed to 8.5

Milestone removed

added ~14281 ~13921 labels

It's listed in https://docs.gitlab.com/ee/workflow/README.html

closed