Introduction tweaks

[glyn]

Move the initial examples into the introduction since the purpose of the examples is introductory.

Mark certain sections as informative (rather than normative).

[glyn]

This works for me. That said, "This section is for information only" might be a less SDO-geeky way of saying this.

That still feels a little vacuous. How about "This section is non-normative."? Too formal?

[glyn]

Merging, to avoid clashes with another PR.