include: More category documentation.

Reference Issue #11847.

include/SDL3/SDL_tray.h

@@ -22,7 +22,11 @@
 /**
  * # CategoryTray
  *
- * System tray menu support.
+ * SDL offers a way to add items to the "system tray" (more correctly called
+ * the "notification area" on Windows). On platforms that offer this concept,
+ * an SDL app can add a tray icon, submenus, checkboxes, and clickable
+ * entries, and register a callback that is fired when the user clicks on
+ * these pieces.
  */
 
 #ifndef SDL_tray_h_