[llvm] [ADT] Document the UniquingSet contracts. NFC (PR #221860)

[Kazu Hirata via llvm-commits]

https://github.com/kazutakahirata approved this pull request.

LGTM modulo some unit test suggestion.  Thank you for documenting when to prefer `UniquingSet` over `FoldingSet`!

https://github.com/llvm/llvm-project/pull/221860