AppKit: Annotate macOS-only standard key bindings

Record that the standard key binding table is not platform-aware.

* CPKeyBinding: Add TODO noting that CPStandardKeyBindings applies macOS bindings on every platform.

The modifier-key conflicts on Windows and Linux are now documented at the source.

AppKit/CPKeyBinding.j

@@ -26,6 +26,12 @@
 @import "CPEvent_Constants.j"
 @import "CPText.j"
 
+// TODO: These are the macOS standard key bindings and are applied on every
+// platform. "@" is always Command, and the Emacs-style Control bindings
+// (^a, ^e, ^k, ^v, ^y, ...) and arrow/Home/End behaviour conflict with
+// Windows and Linux conventions. Provide per-platform tables keyed off
+// CPBrowserIsOperatingSystem, mapping the primary modifier through
+// CPPlatformActionKeyMask.
 CPStandardKeyBindings = {
     @"@.": @"cancelOperation:",