ITS#8565 - Clearly document rootdn requirement for the ppolicy overlay

doc/man/man5/slapo-ppolicy.5

@@ -28,7 +28,8 @@ Note that some of the policies do not take effect when the operation
 is performed with the
 .B rootdn
 identity; all the operations, when performed with any other identity,
-may be subjected to constraints, like access control.
+may be subjected to constraints, like access control.  This overlay
+requires a rootdn to be configured on the database.
 .P
 Note that the IETF Password Policy proposal for LDAP makes sense
 when considering a single-valued password attribute, while