Merge branch 'jc/doc-most-refs-are-not-that-special'

Doc updates. * jc/doc-most-refs-are-not-that-special: docs: MERGE_AUTOSTASH is not that special docs: AUTO_MERGE is not that special refs.h: HEAD is not that special git-bisect.txt: BISECT_HEAD is not that special git.txt: HEAD is not that special