System.Linq.Async: add more detailed docs

[idg10]

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?

dotnet/runtime#79782
#2190
dotnet/docs#44886