revision: make --grep search in notes too if shown

Notes are shown after commit body. From user perspective it looks
pretty much like commit body and they may assume --grep would search
in that part too.

Make it so.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -66,6 +66,9 @@ endif::git-rev-list[]
 	more than one `--grep=<pattern>`, commits whose message
 	matches any of the given patterns are chosen (but see
 	`--all-match`).
++
+When `--show-notes` is in effect, the message from the notes as
+if it is part of the log message.
 
 --all-match::
 	Limit the commits output to ones that match all given --grep,