* 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