docs: explicitly specify mail.use-{tls,ssl} is mutually exclusive (#3368)

Author: aldy505

sentry/config.example.yml

@@ -12,14 +12,15 @@ mail.host: 'smtp'
 # mail.port: 25
 # mail.username: ''
 # mail.password: ''
+# NOTE: `mail.use-tls` and `mail.use-ssl` are mutually exclusive and should not
+#        appear at the same time. Only uncomment one of them.
 # mail.use-tls: false
 # mail.use-ssl: false
 
 # NOTE: The following 2 configs (mail.from and mail.list-namespace) are set
 #       through SENTRY_MAIL_HOST in sentry.conf.py so remove those first if
 #       you want your values in this file to be effective!
 
-
 # The email address to send on behalf of
 # mail.from: 'root@localhost' or ...
 # mail.from: 'System Administrator <root@localhost>'