System.Linq.Async: add more detailed docs

Feedback on initial PR to be handled later:

* "it would be nice to have some of this info in a table to figure out what's good / bad / moved / deprecated"
* there's scope for people to presume that `ToAsyncEnumerable` falls into the same "this was probably a bad idea"
set as `ToEnumerable`, which it doesn't

> Is it worth adding some of the links about the discussion for future maintainers?
>
> https://github.com/dotnet/runtime/issues/79782
> https://github.com/dotnet/reactive/issues/2190
> https://github.com/dotnet/docs/issues/44886