diff-options.txt: document the new "--dirstat" option

This commit adds the documentation for the new option added by 7df7c01
(Add "--dirstat" for some directory statistics, 2008-02-12).

Noticed by Clint Adams, reported through
 http://bugs.debian.org/476437

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -58,6 +58,14 @@ endif::git-format-patch[]
 	number of modified files, as well as number of added and deleted
 	lines.
 
+--dirstat[=limit]::
+	Output only the sub-directories that are impacted by a diff,
+	and to what degree they are impacted.  You can override the
+	default cut-off in percent (3) by "--dirstat=limit".  If you
+	want to enable "cumulative" directory statistics, you can use
+	the "--cumulative" flag, which adds up percentages recursively
+	even when they have been already reported for a sub-directory.
+
 --summary::
 	Output a condensed summary of extended header information
 	such as creations, renames and mode changes.