Add note to docker docs explaining platform support (#9801)

Context is in https://github.com/matrix-org/synapse/issues/9764#issuecomment-818615894.

I struggled to find a more official link for this. The problem occurs when using WSL1 instead of WSL2, which some Windows platforms (at least Server 2019) still don't have. Docker have updated their documentation to paint a much happier picture now given WSL2's support.

The last sentence here can probably be removed once WSL1 is no longer around... though that will likely not be for a very long time.