Clarify how Schema Objs require full-doc parsing (3.2.0 port of 3758)

JSON Schema draft 2020-12 includes numerous keywords that require
parsing the entire document prior to deeming a reference unresolvable.
This makes that more clear and outlines several approaches.

The practice of embedding OpenAPI fragments in other formats is
deemed to have implementation-defined (non-interoperable) behavior,
as the potential complications that might arise are not predictable.

Also provide guidance around fragmentary parsing.

This goes into more detail and uses "undefined" instead of
"implementation-defined" as the behavior is likely to be
incorrect (rather than just a different interpretationof an
ambiguous requirement), and may result in security concerns
as well.