Merge branch 'mg/doc-submodule-status-cached'

"git submodule status" and "git submodule status --cached" show
different things, but the documentation did not cover them
correctly, which has been corrected.

* mg/doc-submodule-status-cached:
  doc: document 'git submodule status --cached'

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