Merge branch 'kh/doc-committer-date-is-author-date'

The "--committer-date-is-author-date" option of "git am/rebase" is
a misguided one.  The documentation is updated to discourage its
use.

* kh/doc-committer-date-is-author-date:
  doc: warn against --committer-date-is-author-date

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

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