docs: Explain the impact of "ntlm auth = disabled" on simple bind forwarding

docs: Explain the impact of "ntlm auth = disabled" on simple bind forwarding

An RODC will forward an LDAP Simple bind, just like any other authentication,
when the password is not present locally.

If the full DC does not support NTLMv2 authentication this forwarded password
will be rejected.  A future Samba version should prefer Kerberos or send the
plaintext, but we can not change the MS Windows behaviour, so we document this.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13879

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>