settings: Try to clarify how AUTH_LDAP_USER_ATTR_MAP works.
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`.

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