clarify and fix typos in lemma 6.10.8
unmarked errata for 13302de
slight clarifications
Merge pull request #581 from mikeshulman/lemma6.10.8

clarify and fix typos in lemma 6.10.8

Someone once said it was annoying how well @mikeshulman could argue his case, but it was even more annoying that he would change his mind in face of a good counter-argument.