That is an explanation of what it means to be non-standard.  That might be good to include also, but I don't think it is a strong enough warning by itself.  The reader needs to be clearly alerted to the fact that "generalized RDF" is a *non-standard* extension of RDF.  Take away the vagueness.  Be blunt and to the point.  This should not be minimized in any way.  It should be in the readers face, so that the reader is forced to make a conscious choice about whether to step outside of standard RDF.

FYI, I just sent a comment to the RDF WG to make the definition of "generalized RDF" clearer in warning that it is non-standard.