docs: Clarify Win32 handle validity

This commit clarifies what is expected by the system for a Win32 handle.
It is expected that the Win32 handle belongs to the current thread. This
is not a breaking change as it was previously necessary for the window
to be considered "valid".

cc rust-windowing/winit#3593

Signed-off-by: John Nunley <dev@notgull.net>

src/windows.rs

@@ -59,6 +59,10 @@ pub struct Win32WindowHandle {
 impl Win32WindowHandle {
     /// Create a new handle to a window.
     ///
+    /// # Safety
+    ///
+    /// It is assumed that the Win32 handle belongs to the current thread. This
+    /// is necessary for the handle to be considered "valid" in all cases.
     ///
     /// # Example
     ///