merge-options.txt: correct wording of --no-commit option

merge-options.txt: correct wording of --no-commit option

The former wording implied that --no-commit would always cause the
merge operation to "pause" and allow the user to make further changes
and/or provide a special commit message for the merge commit.  This
is not the case for fast-forward merges, as there is no merge commit
to create.  Without a merge commit, there is no place where it makes
sense to "stop the merge and allow the user to tweak changes"; doing
that would require a full rebase of some sort.

Since users may be unaware of whether their branches have diverged or
not, modify the wording to correctly address fast-forward cases as well
and suggest using --no-ff with --no-commit if the point is to ensure
that the merge stops before completing.

Reported-by: Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>