Be consistent with normative / non-normative sections

[danielkhan]

See https://lists.w3.org/Archives/Public/public-trace-context/2020Feb/0004.html

A few sections of the document are marked as non-normative, but then their subsections include many direct normative requirements. The Problem Statement and Solution sections (which read to me as non-normative overviews) are not marked as non-normative. I don’t really have any strong feelings about use of non-normative section headers, I’m just passing on that the distinctions in the current spec are confusing.

[plehegar]

(see also #392 (comment) )

[kalyanaj]

We will consider this cleanup for Level 2. Not absolutely required, but more of an editorial fix.