include: Fill in more category documentation.

Reference Issue #11874.

include/SDL3/SDL_metal.h

@@ -23,6 +23,10 @@
  * # CategoryMetal
  *
  * Functions to creating Metal layers and views on SDL windows.
+ *
+ * This provides some platform-specific glue for Apple platforms. Most macOS
+ * and iOS apps can use SDL without these functions, but this API they can be
+ * useful for specific OS-level integration tasks.
  */
 
 #ifndef SDL_metal_h_