Documentation: git fmt-merge-msg does not have to be a script

The fmt-merge-message builtin can be invoked as "git fmt-merge-msg" rather
than through the hard link in GIT_EXEC_PATH.  Although this is unlikely to
confuse most script writers, it should not hurt to make the documentation
a little clearer anyway.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fmt-merge-msg.txt

@@ -18,8 +18,8 @@ Takes the list of merged objects on stdin and produces a suitable
 commit message to be used for the merge commit, usually to be
 passed as the '<merge-message>' argument of 'git-merge'.
 
-This script is intended mostly for internal use by scripts
-automatically invoking 'git-merge'.
+This command is intended mostly for internal use by scripts
+automatically invoking 'git merge'.
 
 OPTIONS
 -------

Documentation/git-merge.txt

@@ -28,9 +28,10 @@ OPTIONS
 include::merge-options.txt[]
 
 -m <msg>::
-	The commit message to be used for the merge commit (in case
-	it is created). The 'git-fmt-merge-msg' script can be used
-	to give a good default for automated 'git-merge' invocations.
+	Set the commit message to be used for the merge commit (in
+	case one is created). The 'git fmt-merge-msg' command can be
+	used to give a good default for automated 'git merge'
+	invocations.
 
 <remote>...::
 	Other branch heads to merge into our branch.  You need at