Document scope limitations of concurrent Stream combinators

[reardonj]

• add general documentation to Stream warning about scopes not extending through operations that use Concurrent
• add specific documentation for the couple of methods that are safe

I've taken a stab at documenting the current behavior instead of trying to fix #3076 given how intractable fixing that bug has been.

I ran variations of this code with various combinators to see what the full scope of the problem is:

Deferred[IO, Unit]
      .flatMap { d =>
        Stream
          .bracket(IO.unit)(_ => d.complete(()).void)
          .prefetch
          .evalMap(_ => IO.sleep(1.second) >> d.complete(()))
          .timeout(5.seconds)
          .compile
          .last
      }

Unfortunately, scopes don't seem to propagate for basically any combinator requiring a Concurrent instance. More specifically, these are generally passing stream elements through channels to a concurrently running stream, and the scopes of the original stream aren't held open until the background stream completes.