[PATCH] diff: document diff-filter exclusion

In v1.8.5 days, 7f2ea5f0 (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 <redacted>
---
 Documentation/diff-options.txt | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 3ad6404..073c7e5 100644
--- a/Documentation/diff-options.txt
+++ b/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.