pull: update docs & code for option compatibility with rebasing

pull: update docs & code for option compatibility with rebasing

git-pull.txt includes merge-options.txt, which is written assuming
merges will happen.  git-pull has allowed rebases for many years; update
the documentation to reflect that.

While at it, pass any `--signoff` flag through to the rebase backend too
so that we don't have to document it as merge-specific.  Rebase has
supported the --signoff flag for years now as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>