Add docs re: XMLElementDeserializable for classes
Add docs re: XMLElementDeserializable for classes

Classes versus structs behave differently when returning Self. This
documentation update reflects this distinction.

jpsim/SWXMLHash,jpsim/SWXMLHash,drmohundro/SWXMLHash,jpsim/SWXMLHash,drmohundro/SWXMLHash