Clarify JSON values early in the spec

[glyn]

Fixes https://github.com/ietf-wg-jsonpath/draft-ietf-jsonpath-base/issues/299

[glyn]

@cabo I take your point about this PR not being easy to review. I will think again.

[cabo]

(Let me just add that being explicit about these things is a significant improvement, but the explanations should not assume that the reader has a specific misunderstanding and try to talk them out of it -- that creates new misunderstandings.)

[glyn]

(Let me just add that being explicit about these things is a significant improvement, but the explanations should not assume that the reader has a specific misunderstanding and try to talk them out of it -- that creates new misunderstandings.)

Very helpful thanks. Let's address this change with a few tiny PRs.