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 #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.
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