[Mailer] Added a note about using absolute URLs in mailer

Author: javiereguiluz

mailer.rst

@@ -200,7 +200,8 @@ Use the ``attachFromPath()`` method to attach files that exist on your file syst
         ->attachFromPath('/path/to/documents/privacy.pdf', 'Privacy Policy')
         // optionally you can provide an explicit MIME type (otherwise it's guessed)
         ->attachFromPath('/path/to/documents/contract.doc', 'Contract', 'application/msword')
-        // you can also use an absolute url
+        // you can also use an absolute URL if your PHP config allows getting URLs using fopen()
+        // (this is not recommended because your application may or may not work depending on PHP config)
         ->attachFromPath('http://example.com/path/to/documents/contract.doc', 'Contract', 'application/msword')
     ;