Clarify response fields for default properties in spec.

[CasperWA]

While I am in full support for the overall changes; after making these changes we are not so helpful on the underlying intent with what properties get included or not. The reader is likely to end up with the feeling that one needs to carefully consider each individual Response point in Sec 6 to decipher arbitrary conditions of when things should be included or not.

In reality, our rules are very straightforward:

• If response_fields is not present: return only a small set of default properties.
• If response_fields is present, return only those fields requested.

I think the right place to say this is early in section 4.1, with maybe a callback from 4.2 to say that the same thing applies for single entry endpoints. Or possibly a new subsection. I started to formulate a suggestion, but do not have time to finish it now. (I'll get back to it eventually if no one else makes an attempt.)

Originally posted by @rartino in #203 (comment)