Editorial updates in WCAG 2.1 publication prep

A couple editorial issues that people raised on the list and I changed 
for the publication:

  * The orange borders for proposals are now red and pass the color
    contrast test;
  * The instructions in the intro more clearly indicate that they are
    indicated as "[Proposed]" with brackets, which now works as a search
    term for people wanting to find them that way;
  * There's a brief clause pointing out that there are also proposed
    definitions to look at;
  * Nobody complained about this, but the "Principle", "Guideline", and
    "Success Criterion" at the start of headers, and the "Level" in the
    conformance level indicator, are no longer CSS generated content.
    However, in the TOC those words are not used (just the section
    numbers) because of how the W3C stylesheets rendered it. I'll work
    on solving that for future publications.

Some of these changes are not in the rawgit because they required manual 
edits to the snapshot that will be published. I'll try to get the github 
versions up to snuff soon, it will require a little script work.

Michael