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/ios/window.rs

@@ -142,7 +142,7 @@ impl Inner {
     }
 
     pub fn set_inner_size(&self, _size: Size) {
-        unimplemented!("not clear what `Window::set_inner_size` means on iOS");
+        warn!("not clear what `Window::set_inner_size` means on iOS");
     }
 
     pub fn set_min_inner_size(&self, _dimensions: Option<Size>) {