prod install: Cut premature suggestion to configure auth backends.

The warning here means that the admin can't really act on this yet if
they want to disable email auth, which is likely among admins that
want to make any changes here.  And for admins who don't, this is an
extra thing to read and make a decision about before they can get a
server running.  See #6985.

Conversely, we already discuss auth backends right at the top of the
`prod-customize` doc, which is linked under "Next steps" at the end of
these instructions.

The warning about EmailAuthBackend is important; but we can move it to
the config file right next to the setting, and then it's available
right when it's actionable, which is if the admin is actually thinking
about changing the setting.

docs/prod-install.md

@@ -90,12 +90,6 @@ heading `### MANDATORY SETTINGS`.  These settings include:
   [our guide for outgoing email](prod-email.html) for help configuring
   this.
 
-- If desired, you can also configure additional
-  [authentication backends](prod-authentication-methods.html) while
-  you're editing /etc/zulip/settings.py.  Note, however, that the
-  default (email) backend must be enabled when you complete Step 6
-  (creating an organization) below.
-
 ## Step 4: Test email configuration
 
 [Test your outgoing email configuration](prod-email.html#testing-and-troubleshooting).

zproject/prod_settings_template.py

@@ -89,6 +89,10 @@ EMAIL_USE_TLS = True
 # Enable at least one of the following authentication backends.
 # See http://zulip.readthedocs.io/en/latest/prod-authentication-methods.html
 # for documentation on our authentication backends.
+#
+# The install process requires EmailAuthBackend (the default) to be
+# enabled.  If you want to disable it, do so after creating the
+# initial realm and user.
 AUTHENTICATION_BACKENDS = (
     'zproject.backends.EmailAuthBackend',  # Email and password; just requires SMTP setup
     # 'zproject.backends.GoogleMobileOauth2Backend',  # Google Apps, setup below