std: Update documentation of seek_write on Windows

Currently the documentation of `FileExt::seek_write` on Windows
indicates that writes beyond the end of the file leave intermediate
bytes uninitialized. This commentary dates back to the original
inclusion of these functions in rust-lang#35704 (wow blast from the past!). At
the time the functionality here was implemented using `WriteFile`, but
nowadays the `NtWriteFile` method is used instead. The documentation for
`NtWriteFile` explicitly states:

> If Length and ByteOffset specify a write operation past the current
> end-of-file mark, NtWriteFile automatically extends the file and updates
> the end-of-file mark; any bytes that are not explicitly written between
> such old and new end-of-file marks are defined to be zero.

This commentary has had a downstream impact in the `system-interface`
crate where it tries to handle this by explicitly writing zeros, but I
don't believe that's necessary any more. I'm sending a PR upstream here
to avoid future confusion and codify that zeros are written in the
intermediate bytes matching what Windows currently provides.

Author: alexcrichton

library/std/src/os/windows/fs.rs

@@ -59,7 +59,7 @@ pub trait FileExt {
     /// function, it is set to the end of the write.
     ///
     /// When writing beyond the end of the file, the file is appropriately
-    /// extended and the intermediate bytes are left uninitialized.
+    /// extended and the intermediate bytes are set to zero.
     ///
     /// Note that similar to `File::write`, it is not an error to return a
     /// short write. When returning from such a short write, the file pointer