Merge branch 'bw/push-submodule-only'

Add missing documentation update to a recent topic.

* bw/push-submodule-only:
completion: add completion for --recurse-submodules=only
doc: add doc for git-push --recurse-submodules=only

diff --cc Documentation/git-push.txt
Simple merge

diff --cc contrib/completion/git-completion.bash
Simple merge