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>

Documentation/merge-options.txt

@@ -3,9 +3,14 @@
 	Perform the merge and commit the result. This option can
 	be used to override --no-commit.
 +
-With --no-commit perform the merge but pretend the merge
-failed and do not autocommit, to give the user a chance to
-inspect and further tweak the merge result before committing.
+With --no-commit perform the merge and stop just before creating
+a merge commit, to give the user a chance to inspect and further
+tweak the merge result before committing.
++
+Note that fast-forward updates do not create a merge commit and
+therefore there is no way to stop those merges with --no-commit.
+Thus, if you want to ensure your branch is not changed or updated
+by the merge command, use --no-ff with --no-commit.
 
 --edit::
 -e::