docs: Stop referring to the Quartz backend

Its MacOS now.

gdk/x11/gdkmain-x11.c

@@ -68,10 +68,10 @@
  *     }
  *   else
  * #endif
- * #ifdef GDK_WINDOWING_QUARTZ
- *   if (GDK_IS_QUARTZ_DISPLAY (display))
+ * #ifdef GDK_WINDOWING_MACOS
+ *   if (GDK_IS_MACOS_DISPLAY (display))
  *     {
- *       // make Quartz-specific calls here
+ *       // make MacOS-specific calls here
  *     }
  *   else
  * #endif

testsuite/gtk/autotestkeywords.cc

@@ -4,9 +4,9 @@
 #include <gdk/broadway/gdkbroadway.h>
 #endif
 
-#ifdef GDK_WINDOWING_QUARTZ
+#ifdef GDK_WINDOWING_MACOS
 #if HAVE_OBJC
-#include <gdk/quartz/gdkquartz.h>
+#include <gdk/macos/gdkmacos.h>
 #endif
 #endif