Allows setting the Timer wait_time in start method.

Allows shortening the two line method of Timer.set_wait_time
followed by Timer.start set wait_time as a parameter to
Timer.start. Also modifies the class documentation.

Fixes: #18107

scene/main/timer.h

@@ -64,7 +64,7 @@ public:
 	void set_autostart(bool p_start);
 	bool has_autostart() const;
 
-	void start();
+	void start(float p_time = -1);
 	void stop();
 
 	void set_paused(bool p_paused);