Remove the comment about FILE_FLAG_OVERLAPPED.

There may eventually be something to say about `FILE_FLAG_OVERLAPPED` here,
however this appears to be independent of the other changes in this PR,
so remove them from this PR so that it can be discussed separately.

library/std/src/os/windows/io/raw.rs

@@ -56,8 +56,6 @@ pub trait FromRawHandle {
     ///
     /// The `handle` passed in must:
     ///   - be a valid an open handle,
-    ///   - be a handle opened for synchronous I/O, *without* the
-    ///     `FILE_FLAG_OVERLAPPED` flag, and
     ///   - be a handle for a resource that may be freed via [`CloseHandle`]
     ///     (as opposed to `RegCloseKey` or other close functions).
     ///