Annotated enums and extended validation

This tidies up the increasingly long Schema Object section and adds
explanations of what I've been calling "extended validation",
including validating `readOnly` and `writeOnly`, and also using
a `oneOf` or `anyOf` with `const` plus annotations for enumerated
types with additional information.

There are many OAS issues/discussions where we have recommended
these techniques, so it makes sense to include them in 3.1.1 where
draft 2020-12's formal collection of annotations enables tools
to build support for them.