Optimize ldap modules auth note in a doc fragment (#9486)

Optimize ldap auth doc fragment

plugins/doc_fragments/ldap.py

@@ -13,6 +13,10 @@ __metaclass__ = type
 class ModuleDocFragment(object):
     # Standard LDAP documentation fragment
     DOCUMENTATION = r"""
+notes:
+  - The default authentication settings will attempt to use a SASL EXTERNAL bind over a UNIX domain socket. This works well with the default Ubuntu
+    install for example, which includes a C(cn=peercred,cn=external,cn=auth) ACL rule allowing root to modify the server configuration. If you need
+    to use a simple bind to access your server, pass the credentials in O(bind_dn) and O(bind_pw).
 options:
   bind_dn:
     description: