Appendix on converting data types to strings (3.2.0 port of 3840)

It's very unclear how numbers, booleans, and other non-UTF-8-string
values are converted to strings, particularly for the form media types.
This adds a brief appendix that acknowledges the lack of standardization,
and points to resources for the few cases that do have specifications.

It highlights concerns with relying on certain JSON Schema keywords
or values for serialization, and suggests defining schemas of
type string and requiring applications to perform the conversion
prior to schema validation as a way to control the results.

This also clarifies that schema validation occurs before serialization.

Also add note about RFC6570 type conversions.  The spec doesn't
address it, but implementations often have their own rules.