Wsl: Mention explicitly how WSL 2 distros run as containers under a single kernel

[cerebrate]

This particular detail of the WSL architecture makes it behave in ways different from a "generic" Linux installation (e.g., kernel threads do not show up in a process listing, et. sim.), and these ways are unexpected if you don't know what's going on under the hood. And hence it would be useful to have that in the docs.

(Initially reported at https://github.com/microsoft/WSL/issues/4622 .)

[craigloewen-msft]

Agreed! I aim to add a 'How WSL2' works doc as we approach general release and will make sure this detail is added in there.