doc: git-pull: clarify options for integrating remote branch

doc: git-pull: clarify options for integrating remote branch

From user feedback:

- One user is confused about the current default ("I was convinced that
  the git default was still to merge on pull")
- One user is confused about why "git fetch" isn't mentioned earlier
- One user says they always forget what the arguments to `git pull` are
  and that it's not immediately obvious that `--no-rebase` means "merge"
- One user wants `--ff-only` to be mentioned

Resolve this by listing the options for integrating the the remote
branch. This should help users figure out at a glance which one they
want to do, and make it clearer that --ff-only is the default.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>