api: finish some thoughts in callbacks javadoc

runelite-api/src/main/java/net/runelite/api/Client.java

@@ -48,10 +48,14 @@ import net.runelite.api.widgets.WidgetInfo;
 public interface Client extends GameEngine
 {
 	/**
-	 * The client invokes these callbacks to communicate to
+	 * The injected client invokes these callbacks to send events to us
 	 */
 	Callbacks getCallbacks();
 
+	/**
+	 * The injected client invokes these callbacks for scene drawing, which is
+	 * used by the gpu plugin to override the client's normal scene drawing code
+	 */
 	DrawCallbacks getDrawCallbacks();
 
 	void setDrawCallbacks(DrawCallbacks drawCallbacks);