Document that soft bodies should be avoided for gameplay purposes

(cherry picked from commit 94b1ae55995f15520dce252cb8f3fa6f65ff6102)

doc/classes/SoftBody.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		A deformable physics body. Used to create elastic or deformable objects such as cloth, rubber, or other flexible materials.
+		[b]Note:[/b] There are many known bugs in [SoftBody]. Therefore, it's not recommended to use them for things that can affect gameplay (such as a player character made entirely out of soft bodies).
 	</description>
 	<tutorials>
 		<link>https://docs.godotengine.org/en/3.4/tutorials/physics/soft_body.html</link>