Particles: Make collision with objects optional (#7682)

Also set it to false for node dig particles, as they are often created
and high in number.

Improve particle documentation.

src/client/clientevent.h

@@ -82,6 +82,7 @@ struct ClientEvent
 			f32 size;
 			bool collisiondetection;
 			bool collision_removal;
+			bool object_collision;
 			bool vertical;
 			std::string *texture;
 			struct TileAnimationParams animation;
@@ -103,6 +104,7 @@ struct ClientEvent
 			f32 maxsize;
 			bool collisiondetection;
 			bool collision_removal;
+			bool object_collision;
 			u16 attached_id;
 			bool vertical;
 			std::string *texture;