Document sendemail.envelopesender configuration

Signed-off-by: Ask Bjørn Hansen <ask@develooper.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -179,6 +179,9 @@ user is prompted for a password while the input is masked for privacy.
 	This is useful if your default address is not the address that is
 	subscribed to a list. If you use the sendmail binary, you must have
 	suitable privileges for the -f parameter.
+	Default is the value of the 'sendemail.envelopesender' configuration
+	variable; if that is unspecified, choosing the envelope sender is left
+	to your MTA.
 
 --to::
 	Specify the primary recipient of the emails generated.