diff: document diff-filter exclusion

In v1.8.5 days, 7f2ea5f (diff: allow lowercase letter to specify
what change class to exclude, 2013-07-17) taught the "--diff-filter"
mechanism to take lowercase letters as exclusion, but we forgot to
document it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -399,6 +399,9 @@ ifndef::git-format-patch[]
 	paths are selected if there is any file that matches
 	other criteria in the comparison; if there is no file
 	that matches other criteria, nothing is selected.
++
+Also, these upper-case letters can be downcased to exclude.  E.g.
+`--diff-filter=ad` excludes added and deleted paths.
 
 -S<string>::
 	Look for differences that change the number of occurrences of