default_settings: Add explanatory comment to "sends_email_by_ip".

Add "5 emails per day" comment
to indicate what (86400, 5) inside "sends_email_by_ip" means.

zproject/default_settings.py

@@ -320,6 +320,7 @@ DEFAULT_RATE_LIMITING_RULES = {
     # sending of an email, restricting the number per IP address. This
     # is a general anti-spam measure.
     "sends_email_by_ip": [
+        # 5 emails per day.
         (86400, 5),
     ],
     # Limits access to uploaded files, in web-public contexts, done by