#162 follow-ups: NaN position in sort/min/max, document put mutation, difftest test_env #164

From Motoko's #162 review.

1. Where NaN ends up after sort or min/max depends on its position: (min (list n 1.0)) gives NaN but (min (list 1.0 n)) gives 1. Document it, or order NaN last.
2. Document that put mutates the dict and returns the same dict.
3. difftest's test_env.howl fails because the interpreter leg has no env grant. Fix it or skip it explicitly.