CORBA23 — Custom Marshaling clarification

Summary: In the core 2.3a chapters, page 5-11, section 5.5.1, last paragraph needs
to be clarified or deleted. This paragraph originally referred to the
"streaming policy" registration APIs that used to be specified here but
have since been deleted.

Do we still intend to allow language mappings to provide alternative
mechanisms for custom marshaling as well as the standard mechanism of
having the valuetype implementation provide the CustomMarshal methods
marshal and unmarshal? If not, this paragraph should be deleted. If so,
the last part of the paragraph should be reworded, for example "... to
support other ways for value types to implement custom marshaling".

Incorporate change in 2.4 and close issue