Spell out duplicate hook locations.

props DrewAPicture.
fixes #25658.

Built from https://develop.svn.wordpress.org/trunk@25868


git-svn-id: http://core.svn.wordpress.org/trunk@25868 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-mail.php

@@ -10,6 +10,7 @@
 /** Make sure that the WordPress bootstrap has run before continuing. */
 require(dirname(__FILE__) . '/wp-load.php');
 
+/** This filter is documented in wp-admin/options.php */
 if ( ! apply_filters( 'enable_post_by_email_configuration', true ) )
 	wp_die( __( 'This action has been disabled by the administrator.' ) );