v3.1: sync percent-encoding improvements from 3.2?

[ralfhandl]

@handrews very diligently improved the rules and explanations around percent-encoding in Appendices D and E of the 3.2 spec.

Should these improvements be downported to 3.1 (were appropriate)?

[handrews]

@ralfhandl they went into 3.1.2 first (PRs #4821 and #4825), and were then propagated to 3.2 with other fixes (PR #4862). So this is already done.