Improve section 5

[relu91]

From my part, we can deal with improving the section 5 vs appendix A in a new issue and merge this PR.

Originally posted by @zolkis in #524 (review)

Keep in mind this comment.

[relu91]

@zolkis do you remember exactly what we should improve? I'll also review the PR linked to extract concrete action points.

[zolkis]

IIRC it was about to keep examples using the Fetch API (and validation), as well as an ed.note about extending options for requestThingDescription(). The latter is there, the former is mentioned in text, good enough for me (not necessary to include full validation example outside of the strict scope of this API spec -- though it could be useful).

[relu91]

Scripting Task Force decided to close this.