Add Castor XSD information to reference docs

Update the Spring OXM reference documentation to include changes
introduced in CastorMarshaller, specifically around CastorMarshaller
XSD configuration.

Issue: SPR-8509
Merge pull request #243 from jmnarloch/SPR-8509

# By Jakub Narloch
* SPR-8509:
  Add Castor XSD information to reference docs