Update LDAP KDB module documentation

Update LDAP KDB module documentation

Remove ldapbackend.rst, as it is largely redundant with conf_ldap.rst.

Simplify conf_ldap.rst, using kerberos.openldap.ldif (added by ticket
8529) and removing unnecessary command arguments.  Mention the
possibility of using SASL authentication (added by ticket 7944) as an
alternative to binding with DN and password.  Remove unnecessary
access rights.

In kdc_conf.rst, remove ldap_servers from the list of relations read
from [dbdefaults], as it is only read from the realm's database
configuration section.

In kdb5_ldap_util.rst, document "-r" as a global parameter, as it
applies in some fashion to all commands.  Make the same changes to the
kdb5_ldap_util usage message, and make it fit within 80 columns.

Reported by Dilyan Palauzov.

ticket: 8831 (new)
tags: pullup
target_version: 1.17-next