PrivateCredentialPermission.SELF should not be exposed outside of Policy impl

currently, PrivateCredentialPermission, and javax.security.auth.Policy
both document support for a special feature for "SELF" permissions.
while this functionality is useful, it belongs in the actual implementation
for the Policy, and not as part of the permission class nor the abstract
Policy class.

therefore, all support and documentation for the SELF permissions should
occur in com.sun.security.auth.PolicyFile.