settings: Try to clarify how AUTH_LDAP_USER_ATTR_MAP works.

This comment didn't really explain things unless you were looking at
the code, and/or had a strong enough association for what "cn" means
that it was obvious what must be meant.  Maybe this will be clearer.

There is one other meaningful key, which is optional: "short_name",
for which I guess a typical value if supplied would be "uid" or
"userid".  I'm not sure we really do much with a user's `short_name`,
though, so didn't add a comment for it.  When this key is omitted,
we set the user's `short_name` to the same thing as `full_name`.

zproject/prod_settings_template.py

@@ -410,7 +410,7 @@ LDAP_EMAIL_ATTR = None  # type: Optional[str]
 
 # This map defines how to populate attributes of a Zulip user from LDAP.
 AUTH_LDAP_USER_ATTR_MAP = {
-    # Populate the Django user's name from the LDAP directory.
+    # full_name is required; common values include "cn" or "displayName".
     "full_name": "cn",
 }