Merge branch 'mm/keyid-docs'

Very small number of options take a parameter that is optional
(which is not a great UI element as they can only appear at the end
of the command line).  Add notice to documentation of each and
every one of them.

* mm/keyid-docs:
Documentation: explain optional arguments better
Documentation/grep: fix documentation of -O
Documentation: use 'keyid' consistently, not 'key-id'

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

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

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

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