v3.1.2: New percent-encoding section (partial fix)

[handrews]

This is the starting point for fixing issue #4813. It has taken me over a week of research and multiple write-up attempts to distill this down and figure out how to give guidance that is both readable and sufficiently concrete.

What is not apparent from this PR is that the specs contradict each other- see #4813 for some of the details (although I did not put everything even there). I realized that if we talk about percent-decoding compliance, everything gets much easier. This make the perecent-encoding requirements a bit ambiguous, but that is inherent in the mess of conflicting requirements, and the way that different specs can add reserved behavior or scheme-specific rules.

The rest of the guidance in the spec won't need too much changing, but I'd like to see if this new section works for folks before tweaking all of the appendixes and examples to align with it. A bit of this is duplicated in Appendix E, but I'll take care of that when updating Appendix E once this is approved.

My goal here is to get the essential guidance up in the early part of the spec, and leave the really fiddly details in the appendix.

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request