doc/git-send-email: mention sendemail.cc config variable

This variable was added in 5f8b9fc (git-send-email: add a new
sendemail.cc configuration variable, 2008-04-27), but is not yet refered
to by the documentation.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -34,6 +34,7 @@ The --bcc option must be repeated for each user you want on the bcc list.
 
 --cc::
 	Specify a starting "Cc:" value for each email.
+	Default is the value of 'sendemail.cc'.
 +
 The --cc option must be repeated for each user you want on the cc list.