settings: Revise comments on SMTP / outgoing email settings.

Add a clear heading, and use fewer words and simpler sentences.  Also
explain the password thing a bit more, and put that more inline next
to the username.

Also, on checking the Django docs, the default for EMAIL_USE_TLS
is False and for EMAIL_PORT is 25.  So most admins, certainly any that
are using an SMTP service on the public Internet (that is at all
decently run), will need to set those settings.  Mention that.

zproject/prod_settings_template.py

@@ -8,8 +8,8 @@ from typing import Optional
 #
 # Developer documentation on the Zulip settings system is available at:
 #   https://zulip.readthedocs.io/en/latest/subsystems/settings.html
-#
+
-### MANDATORY SETTINGS
+## MANDATORY SETTINGS
 #
 # These settings MUST be set in production. In a development environment,
 # sensible default values will be used.
@@ -32,34 +32,31 @@ EXTERNAL_HOST = 'zulip.example.com'
 # 'Zulip Support <support@example.com>'.
 ZULIP_ADMINISTRATOR = 'zulip-admin@example.com'
 
-# Configure the outgoing Email (aka SMTP) server below. You will need
+
-# working SMTP to complete the installation process, in addition to
+### OUTGOING EMAIL (SMTP) SETTINGS
-# sending email address confirmations, missed message notifications,
-# onboarding follow-ups, and other user needs. If you do not have an
-# SMTP server already, we recommend services intended for developers
-# such as Mailgun.  Detailed documentation is available at:
 #
+# Zulip needs to be able to send email (that is, use SMTP) so it can
+# confirm new users' email addresses and send notifications.
+#
+# If you don't already have an SMTP provider, free ones are available.
+#
+# For more details, including a list of free SMTP providers and
+# advice for troubleshooting, see the Zulip documentation:
 #   https://zulip.readthedocs.io/en/latest/production/email.html
-#
+
-# To configure SMTP, you will need to complete the following steps:
+# EMAIL_HOST and EMAIL_HOST_USER are generally required.
-#
-# (1) Fill out the outgoing email sending configuration below.
-#
-# (2) Put the SMTP password for EMAIL_HOST_USER in
-# /etc/zulip/zulip-secrets.conf as e.g.:
-#
-#    email_password = abcd1234
-#
-# You can quickly test your sending email configuration using:
-#   su zulip
-#   /home/zulip/deployments/current/manage.py send_test_email username@example.com
-#
-# A common problem is hosting provider firewalls that block outgoing SMTP traffic.
-#
 #EMAIL_HOST = 'smtp.example.com'
 #EMAIL_HOST_USER = ''
-#EMAIL_PORT = 587
+
+# Passwords and secrets are not stored in this file.  The password
+# for user EMAIL_HOST_USER goes in `/etc/zulip/zulip-secrets.conf`.
+# In that file, set `email_password`.  For example:
+#   email_password = abcd1234
+
+# EMAIL_USE_TLS and EMAIL_PORT are required for most SMTP providers.
 #EMAIL_USE_TLS = True
+#EMAIL_PORT = 587
+
 
 ## OPTIONAL SETTINGS