LDAP authentication documentation: add paragraph about group creation required.

[nicobbg]

Change Type

[X] Addition
[ ] Correction
[ ] Deprecation
[ ] Cleanup (formatting, typos, etc.)

Proposed Changes

On the netbox ldap configuration page it would be great to explain that the three users groups (is_active, is_staff, is_superuser) must exist in the LDAP directory otherwise authentication will fail while trying to retrieve the user group mapping.

Reason; Netbox is trying to retrieve permissions using an LDAP compareRequest, however if one of the group is not present in the LDAP directory, the LDAP server may reply with a success response instead of compareResponse. Because of the implementation of the Python ldap module, such a response would lead to an exception and authentication will fail.