file module documentation: no references to #file_descriptor{}

[Davide Marquês]

Hello!

The documentation for the file module seems to be
incomplete in that it doesn't include any references
to the support for #file_descriptor{} (as returned by
ram_file:open/2) in the functions that normally receive
an io_device().

As it stands, someone (coff coff) might wrongfully
assume that ram_file:open/2 also returns an
io_device() and try to use it in other places where
an io_device() is expected (failing of course). :)

Cheers,
:Davide