[docs] [issue30803] Truth value of sets not properly documented

[Peter Thomassen]

Peter Thomassen added the comment:

I like your most recent suggestion, and updated the PR after fixing a typo ('Fractions') and making it more complete (complex numbers).

Let me know if anything else is needed.

(A mapping is not a specialized set, at least as far as typing is concerned: `isinstance({}, set)` is false. Semantically, they may be related, but the question here is whether the types are actually related.)

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