[doc] Mention __slots__ behavior in weakref.rst (GH-21061)

It took me longer than I expected to figure out why a random class
I dealt with didn't support weak references. I believe this addition
will make the __slots__/weakref interaction more discoverable to people
having troubles with this. (Before this patch __slots__ was not
mentioned in weakref documentation even once).

Co-authored-by: Łukasz Langa <lukasz@langa.pl>