convert-standard-filename's doc

[Juanma Barranquero <jmbarranquero@wke.es>]

Four years ago, the docstring for convert-standard-filename (in files.el)
was changed, but the ones in w32-fns.el and dos-fns.el were not.

Now, it's trivial to fix them to be identical again. But I wonder
whether

    "Convert a standard file's name to something suitable for the current OS.
  This function's standard definition is trivial; it just returns the argument.
  However, on some systems, the function is redefined with a definition
  that really does change some file names to canonicalize certain
  patterns and to guarantee valid names."

is a good description for a function which really *does* something. In
other words, I'd change the docstring for the w32-fns.el version to
read:

    "Convert a standard file's name to something suitable for Windows.

  Returns FILENAME after replacing invalid characters with ! and
  converting directory separators to the Windows style (if needed).
  It also turns Cygwin-style /cygdrive/x/ pathnames to x:/ paths."

(and add also a suitable docstring for the dos-fns.el version, of course).

                                                                Juanma