Property: naming and manual clarifiations

- Add a bit more text about that is expected of the user or
  OSSL_METHOD_STOREs.
- Clarify what a method and what a numeric identity are.
- Change all mentions of 'implementation' and 'result' to 'method'.

To clarify further: OpenSSL has used the term 'method' for structures
that mainly contains function pointers.  Those are the methods that
are expected to be stored away in OSSL_METHOD_STOREs.  In the end,
however, it's the caller's responsibility to define exactly what they
want to store, as long as its 'methods' are associated with a numeric
identity and properties.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8265)