update the doc for Lwt_unix.{recv,send}_msg
update the doc for Lwt_unix.{recv,send}_msg

Ignore-this: 3a9f33045b388e74ad8a14661301e767

darcs-hash:20130408205322-c41ad-01b5579b5b1c30442c7b03b271c99b471de1b6c1

gfxmonk/lwt,mfp/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,rgrinberg/lwt,ocsigen/lwt,rneswold/lwt,mirage/lwt,fdopen/lwt,ocsigen/lwt,c-cube/lwt,rgrinberg/lwt,mirage/lwt,rgrinberg/lwt,rgrinberg/lwt,c-cube/lwt,fdopen/lwt,janestreet/lwt-async,gfxmonk/lwt,rneswold/lwt,janestreet/lwt-async,mfp/lwt