Document the --attach flag.

Documentation/git-format-patch.txt

@@ -9,7 +9,7 @@ git-format-patch - Prepare patches for e-mail submission
 SYNOPSIS
 --------
 [verse]
-'git-format-patch' [-n | -k] [-o <dir> | --stdout] [-s] [-c]
+'git-format-patch' [-n | -k] [-o <dir> | --stdout] [--attach] [-s] [-c]
 		 [--diff-options] <his> [<mine>]
 
 DESCRIPTION
@@ -60,6 +60,9 @@ OPTIONS
 	standard output, instead of saving them into a file per
 	patch and implies --mbox.
 
+--attach::
+	Create attachments instead of inlining patches.
+
 
 EXAMPLES
 --------