Add docs to multiple assignees for merge requests feature

Update quick-action docs

The following discussion from !10161 (merged) should be addressed:

• @nick.thomas started a discussion: (+1 comment)

  I'm a bit confused about the difference between /assign and /reassign here.

  The docs state that both can take either a single user, or multiple users: https://docs.gitlab.com/ee/user/project/quick_actions.html#quick-actions-for-issues-and-merge-requests

  They also state that /reassign @user1 @user2 is available for merge requests, and that /assign @user1 @user2 is not.

  It's not stated in the docs, but /unassign @user1 @user2 seems to be supported as well.

  At a minimum, I think we also need specs for the /assign and /unassign cases covering merge requests, and a docs update. I'd also love to know what /reassign actually does ^^. How does it differ from /assign ?

• Update API docs

• Update general docs