Document mailto override.

doc/utils.md

@@ -203,6 +203,13 @@ predicate.
 
 A simple wrapper module to send emails from UNIX system.
 
+    (*mailto-override* [email])
+
+* ```email``` - email address string
+
+If this parameter is non-```#f```, all outgoing emails are actually
+sent to the address stored within.
+
     (send-mail body-lines [#:from from] [#:to to] [#:subject subject])
 
 * ```body-lines``` - lines of the email