doc: link GPUParticles2D to the 2D Particles demo

Add a comment in the link to the Dodge The Creeps demo too. That demo only uses
particles for one thing (to leave a trail when the player moves), they're not
interacting with anything else in the demo; it took me a bit of searching to
find where they were used.

(cherry picked from commit 6db9752f23d73f4992bd32ecc73261d9f849c5f4)

doc/classes/Particles2D.xml

@@ -13,7 +13,8 @@
 	</description>
 	<tutorials>
 		<link title="Particle systems (2D)">$DOCS_URL/tutorials/2d/particle_systems_2d.html</link>
-		<link title="2D Dodge The Creeps Demo">https://godotengine.org/asset-library/asset/515</link>
+		<link title="2D Particles Demo">https://godotengine.org/asset-library/asset/118</link>
+		<link title="2D Dodge The Creeps Demo (uses GPUParticles2D for the trail behind the player)">https://godotengine.org/asset-library/asset/515</link>
 	</tutorials>
 	<methods>
 		<method name="capture_rect" qualifiers="const">