UserInputHandler: add API docs

src/UserInputHandler.hxx

@@ -10,11 +10,27 @@
 enum class Command : unsigned;
 struct Point;
 
+/**
+ * Handler for input events from the user, called by #AsyncUserInput.
+ */
 class UserInputHandler {
 public:
+	/**
+	 * A hot key for a #Command was pressed.
+	 *
+	 * @return false if ncmpc will quit
+	 */
 	virtual bool OnCommand(Command cmd) noexcept = 0;
 
 #ifdef HAVE_GETMOUSE
+	/**
+	 * An input event from the mouse was detected.
+	 *
+	 * @param p the position of the mouse cursor on the screen
+	 * @param bstate the state of all mouse buttons
+	 *
+	 * @return false if ncmpc will quit
+	 */
 	virtual bool OnMouse(Point p, mmask_t bstate) noexcept = 0;
 #endif
 };