Merge branch 'jc/am-whitespace-doc'

"git am --help" now tells readers what actions are available in
"git am --whitespace=<action>", in addition to saying that the
option is passed through to the underlying "git apply".

* jc/am-whitespace-doc:
  doc: add shortcut to "am --whitespace=<action>"

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

diff --cc apply.c
Simple merge