settings: Clean up EMAIL_HOST documentation a bit.

Gmail is a bad example for outbound email; use a generic example.

Also leave the `= None` default out of the config file, as it's
redundant with DEFAULT_SETTINGS in our internal settings.py ; and
explain in the latter why we don't mention the other SMTP settings.

zproject/settings.py

@@ -119,10 +119,14 @@ DEFAULT_SETTINGS = {
     'ALLOWED_HOSTS': [],
 
     # Basic email settings
-    'EMAIL_HOST': None,
     'NOREPLY_EMAIL_ADDRESS': "noreply@" + EXTERNAL_HOST.split(":")[0],
     'PHYSICAL_ADDRESS': '',
 
+    # SMTP settings
+    'EMAIL_HOST': None,
+    # Other settings, like EMAIL_HOST_USER, EMAIL_PORT, and EMAIL_USE_TLS,
+    # we leave up to Django's defaults.
+
     # Google auth
     'GOOGLE_OAUTH2_CLIENT_ID': None,