Clarify docs of Lwt_io.(with|open)_file
Clarify docs of Lwt_io.(with|open)_file

Resolves #587

[skip ci]

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