macOS: Add note about high-resolution workaround in QCocoaGLContext::makeCurrent

Change-Id: I0b82ef95c1a058586e8005665e1e2cab3f975833
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

src/plugins/platforms/cocoa/qcocoaglcontext.mm

@@ -371,6 +371,8 @@ bool QCocoaGLContext::makeCurrent(QPlatformSurface *surface)
     // convertSizeToBacking and backingScaleFactor APIs. A typical result of this is that Qt
     // will display a quarter of the window content when running in a virtual machine.
     if (!m_didCheckForSoftwareContext) {
+        // FIXME: This ensures we check only once per context,
+        // but the context may be used for multiple surfaces.
         m_didCheckForSoftwareContext = true;
 
         const GLubyte* renderer = glGetString(GL_RENDERER);