include: Filled in some missing documentation.

include/SDL3/SDL_keyboard.h

@@ -39,6 +39,17 @@
 extern "C" {
 #endif
 
+/**
+ * This is a unique ID for a keyboard for the time it is connected to the
+ * system, and is never reused for the lifetime of the application.
+ *
+ * If the keyboard is disconnected and reconnected, it will get a new ID.
+ *
+ * The ID value starts at 1 and increments from there. The value 0 is an
+ * invalid ID.
+ *
+ * \since This datatype is available since SDL 3.0.0.
+ */
 typedef Uint32 SDL_KeyboardID;
 
 /**