Trivial (hopefully) fixes to docs

This PR is meant to be very easy to merge. If there's something you don't like, let me know and I'll back it out. I added a few clarifying notes that shouldn't be too controversial and removed a reference to the nonexistent `(?)` operator. Dict and Set equality is unreliable as a point of fact and we should document it.

````elm
import Set as S
import Text (asText)

main = asText <| S.fromList [1,2] == S.fromList [2,1]
````
Again, let me know if any particular change is troublesome. Thanks.