Clarify discriminator + oneOf/anyOf/allOf usage (3.1.1)

This moves some guidance up to the fixed fields section where
it is more obvious, and explicitly designates other configurations
as having undefined behavior.

It also creates subsections to organize the different topics, pulls
key guidance out of the examples and up into those sections,
and provides clarification on the ambiguity of names and URIs.

Finally, it incorporates ideas from @jdesrosiers regarding
the ambiguous `mapping` syntax submitted in a prior PR, but
does so in a way that meets our compatibility requirements
for patch releases.
For the same compatibility reasons, the MUST wording for
requiring the named discriminator property in the schema
was (regrettably) weakened to a "SHOULD but otherwise undefined",
as we have done for other problematic ambiguities.

Co-authored-by: Jason Desrosiers <jdesrosi@gmail.com>