Fix Window platform support documentation

This resolves various problems with the documentation about platform
support on the Window struct.

It also completely removes pointless runtime errors in favor of
consistent no-ops on all platforms that do not support a certain
features.

src/platform_impl/windows/window.rs

@@ -609,7 +609,7 @@ impl Window {
 
     #[inline]
     pub fn set_ime_position(&self, _position: Position) {
-        unimplemented!();
+        warn!("`Window::set_ime_position` is ignored on Windows")
     }
 
     #[inline]