Use uniform description for the '--' option.

All descriptions of the '--' option were the same except for that in
Documentation/git-merge-index.txt.
Signed-off-by: NNikolai Weibull <nikolai@bitwi.se>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-merge-index.txt

@@ -20,7 +20,7 @@ files are passed as arguments 5, 6 and 7.
 OPTIONS
 -------
 --::
-	Interpret all following arguments as filenames.
+	Do not interpret any more arguments as options.
 
 -a::
 	Run merge against all files in the index that need merging.