Generalize email sending logging (#6075)

In ancient times Synapse would only send emails when it was notifying a user about a message they received...

Now it can do all sorts of neat things!

Change the logging so it's not just about notifications.

changelog.d/6075.misc

@@ -0,0 +1 @@
+Change mailer logging to reflect Synapse doesn't just do chat notifications by email now.

synapse/push/mailer.py

@@ -311,7 +311,7 @@ class Mailer(object):
         multipart_msg.attach(text_part)
         multipart_msg.attach(html_part)
 
-        logger.info("Sending email notification to %s" % email_address)
+        logger.info("Sending email to %s" % email_address)
 
         yield make_deferred_yieldable(
             self.sendmail(