Documentation: add a bit about sendemail.to configuration

While there is information about this in the configuration section, it was
missing in the options section.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-send-email.txt b/Documentation/git-send-email.txt
index 4b8ec8a..0554f2b 100644
--- a/Documentation/git-send-email.txt
+++ b/Documentation/git-send-email.txt
@@ -137,6 +137,8 @@
 	Specify the primary recipient of the emails generated.
 	Generally, this will be the upstream maintainer of the
 	project involved.
+	Default is the value of the 'sendemail.to' configuration value;
+	if that is unspecified, this will be prompted for.
 +
 The --to option must be repeated for each user you want on the to list.