HPACK editorial changes

This includes a whole raft of editorial changes, including:
* grammatical corrections
* small and less-small wording changes
* reorganization to make the document a little flatter overall, 
* new introductory text in empty sections
* reformatting and reflowing of text (I've isolated these changes and identified them clearly to aid review)
* some xml2rfc tweaking

Feel free to skip changesets that look off; we can discuss those further.  I don't think that I've done anything controversial here, but I might be wrong.