added documentation (and links to rdfg literature) on identifiers, to docstring
added documentation (and links to rdfg literature) on identifiers, to docstring

yingerj/rdflib,ssssam/rdflib,avorio/rdflib,marma/rdflib,armandobs14/rdflib,yingerj/rdflib,avorio/rdflib,dbs/rdflib,yingerj/rdflib,RDFLib/rdflib,avorio/rdflib,yingerj/rdflib,dbs/rdflib,ssssam/rdflib,armandobs14/rdflib,RDFLib/rdflib,ssssam/rdflib,RDFLib/rdflib,avorio/rdflib,armandobs14/rdflib,marma/rdflib,ssssam/rdflib,RDFLib/rdflib,dbs/rdflib,armandobs14/rdflib,marma/rdflib,dbs/rdflib,marma/rdflib