Merge branch 'pb/complete-and-document-auto-merge-and-friends'

Document more pseudo-refs and teach the command line completion
machinery to complete AUTO_MERGE.

* pb/complete-and-document-auto-merge-and-friends:
  completion: complete AUTO_MERGE
  Documentation: document AUTO_MERGE
  git-merge.txt: modernize word choice in "True merge" section
  completion: complete REVERT_HEAD and BISECT_HEAD
  revisions.txt: document more special refs
  revisions.txt: use description list for special refs