Merge branch 'je/doc-checkout'

Doc updates. * je/doc-checkout: doc: git-checkout: clarify restoring files section doc: git-checkout: split up restoring files section doc: git-checkout: deduplicate --detach explanation doc: git-checkout: clarify `-b` and `-B` doc: git-checkout: clarify `git checkout <branch>` doc: git-checkout: clarify ARGUMENT DISAMBIGUATION doc: git-checkout: clarify intro sentence