docs: improve docs for set ops

[NickCrews]

Fixes #11391

[cpcloud]

LGTM, just some minor style suggestions.

[KronosTheLate]

That looks very precise and clear, thanks for the effort!

[KronosTheLate]

I should have mentioned this earlier, but just came to think of it:

Would it not be more clear to name the distinct argument multiset instead?

[NickCrews]

I think I actually prefer prefer distinct over multiset, to be difficult :). But I don't love distinct either. To me it makes good sense for union(), some amount of sense for intersection(), but not much sense for difference(). If you have other ideas throw them out there.

I think whatever name we go with, it must be the same in all of difference(), union(), and intersection().