Merge branch 'pb/doc-orig-head'

Document ORIG_HEAD a bit more.

* pb/doc-orig-head:
  git-rebase.txt: add a note about 'ORIG_HEAD' being overwritten
  revisions.txt: be explicit about commands writing 'ORIG_HEAD'
  git-merge.txt: mention 'ORIG_HEAD' in the Description
  git-reset.txt: mention 'ORIG_HEAD' in the Description
  git-cherry-pick.txt: do not use 'ORIG_HEAD' in example