Warnings for header and cookie serialization (3.0.4)

This makes serializing cookie paramters and most header parameters
with `schema` and `style` NOT RECOMMENDED.
It is not clear that any `schema`-based serialization for cookies
will produce a correct value (although the reason is sufficiently
obscure that many implementations might ignore it and produce
cookie-compliant output anyway).
With headers, there are numerous pitfalls and only the simplest
scenarios will work properly, although perhaps the warning
here could be reworded to emphasize the safe scenarios more clearly.

The details are relegated to an appendix, because truly, most
people will not want to know.  But recommending against syntactically
legal configurations really does need to be explained in the spec.

Also, don't use

- in: header
  name: Cookie

Because... yeah.