close #487
unmarked errata for 8c6f3e8
mention potential pitfall proving 2.8.1, close #494
clarify uniqueness principle for truncations, close #512
Merge pull request #543 from mikeshulman/truncuniq

clarify uniqueness principle for truncations, close #512
Merge pull request #542 from mikeshulman/thm2.8.1

mention potential pitfall proving 2.8.1, close #494
Merge pull request #541 from mikeshulman/after-lemma-2.1.2

close #487