Documentation/merge-options.txt: group "ff" related options together

The --ff-only option was not described next to --ff and --no-ff options in
"git merge" documentation, even though these three are logically together,
describing how to choose one of three possibilities.

Also the description for '--ff' and '--no-ff' discussed what '--ff' means,
and mentioned '--no-ff' as if it were a side-note to '--ff'.

Make them into three top-level entries and list them together. This way,
it would be more clear that the user can choose one from these three.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -13,13 +13,18 @@ inspect and further tweak the merge result before committing.
 	edit the default merge message.
 
 --ff::
+	When the merge resolves as a fast-forward, only update the branch
+	pointer, without creating a merge commit.  This is the default
+	behavior.
+
 --no-ff::
-	Do not generate a merge commit if the merge resolved as
-	a fast-forward, only update the branch pointer. This is
-	the default behavior of git-merge.
-+
-With --no-ff Generate a merge commit even if the merge
-resolved as a fast-forward.
+	Create a merge commit even when the merge resolves as a
+	fast-forward.
+
+--ff-only::
+	Refuse to merge and exit with a non-zero status unless the
+	current `HEAD` is already up-to-date or the merge can be
+	resolved as a fast-forward.
 
 --log[=<n>]::
 --no-log::
@@ -54,11 +59,6 @@ merge.
 With --no-squash perform the merge and commit the result. This
 option can be used to override --squash.
 
---ff-only::
-	Refuse to merge and exit with a non-zero status unless the
-	current `HEAD` is already up-to-date or the merge can be
-	resolved as a fast-forward.
-
 -s <strategy>::
 --strategy=<strategy>::
 	Use the given merge strategy; can be supplied more than