Updated OWLParser documentation

Updated the OWLParser documentation to reflect the fact that parsers
*add* the axioms of the parsed ontology to an existing ontology
object, rather than making the ontology object contain *exactly* the
axioms of the parsed ontology.  Also cleaned up the links to
OWLManager#makeLoadImportRequest.

This was discussed and suggested in a mailing list thread: 
http://sourceforge.net/mailarchive/message.php?msg_id=31823709