Document use of ldapurl with LDAP simple bind

Hi all,

Our documentation implies that the ldapurl setting in pg_hba is used
for search+bind mode only. It was pointed out to me recently that this
is not true, and if you're dealing with simple bind on a non-standard
scheme or port, then ldapurl makes the HBA easier to read:

    ... ldap ldapurl="ldaps://ldap.example.net:49151" ldapprefix="cn="
ldapsuffix=", dc=example, dc=net"

0001 tries to document this helpful behavior a little better, and 0002
pins it with a test. WDYT?

Thanks,
--Jacob