Improve documentation for --dirstat diff option

Signed-off-by: Heikki Orsila <heikki.orsila@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -59,12 +59,11 @@ endif::git-format-patch[]
 	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.
+	Output the distribution of relative amount of changes (number of lines added or
+	removed) for each sub-directory. Directories with changes below
+	a cut-off percent (3% by default) are not shown. The cut-off percent
+	can be set with "--dirstat=limit". Changes in a child directory is not
+	counted for the parent directory, unless "--cumulative" is used.
 
 --summary::
 	Output a condensed summary of extended header information