[stdune] document Spawn.spawn
[stdune] document Spawn.spawn

The function behaves slightly differently between windows and unix. We
should consider changing it. For now, I just noted the differnce with a
comment.

Signed-off-by: Rudi Grinberg <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@rgrinberg.com>

janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder