Add documentation to `Set` to clarify that types need `hash` defined to serve as elements of a `Set`, not just `isequal`.

[sadish-d]

Here's a possible phrasing:

Elements in a Set are unique, as determined by the elements' definition of isequal and hash.

[nsajko]

The phrasing for Set would ideally match the phrasing for Dict. The current phrasing for Dict is:

Keys are compared with isequal and hashed with hash.

IMO your phrasing is better, though, as it's perhaps not clear what "compared" and "hashed" mean without knowing about the implementation.

[sadish-d]

There is a sentence in the current documentation of Set that says:

Elements in a Set are unique, as determined by the elements' definition of isequal.

I was just suggesting an edit to that. If I understand correctly, Sets are implemented as keys of a dictionary. I'm not sure if we should get into the implementation in the documentation. If yes, the documentation could use a bit more of a rewrite maybe.

[nsajko]

I agree with you, I'm just saying we should have similar phrasing in both doc strings.