Use io::Result in prototype documentation

[xavierd]

For a newcomer, there is no reason to expect the return value of any I/O function (ie: std::io::Result) to be any different from std::result::Result as they are both shown to be Result in the documentation.

One easy way to disambiguate this would be to show the prototype to use io::Result instead of Result. For instance, the open function is shown as being:

pub fn open<P: AsRef<Path>>(path: P) -> Result<File>

It should be:

pub fn open<P: AsRef<Path>>(path: P) -> io::Result<File>

[jyn514]

Duplicate of #74924