Lwt_unix: document waitpid blocking on Windows
Lwt_unix: document waitpid blocking on Windows

See #494.

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ocsigen/lwt,ocsigen/lwt,c-cube/lwt,c-cube/lwt,rneswold/lwt,rneswold/lwt,ocsigen/lwt,rneswold/lwt,c-cube/lwt