include: More category documentation.

Reference Issue #11847.

include/SDL3/SDL_touch.h

@@ -22,7 +22,18 @@
 /**
  * # CategoryTouch
  *
- * SDL touch management.
+ * SDL offers touch input, on platforms that support it. It can manage
+ * multiple touch devices and track multiple fingers on those devices.
+ *
+ * Touches are mostly dealt with through the event system, in the
+ * SDL_EVENT_FINGER_DOWN, SDL_EVENT_FINGER_MOTION, and SDL_EVENT_FINGER_UP
+ * events, but there are also functions to query for hardware details, etc.
+ *
+ * The touch system, by default, will also send virtual mouse events; this can
+ * be useful for making a some desktop apps work on a phone without
+ * significant changes. For apps that care about mouse and touch input
+ * separately, they should ignore mouse events that have a `which` field of
+ * SDL_TOUCH_MOUSEID.
  */
 
 #ifndef SDL_touch_h_