Improve documentation related to physics collision shapes

- Document using convex decomposition in the editor.
- Mention that "trimesh" is synonymous with a concave collision shape.
- Add performance hints for each collision shape.

doc/classes/SeparationRayShape3D.xml

@@ -1,10 +1,11 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="SeparationRayShape3D" inherits="Shape3D" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
 	<brief_description>
-		Separation ray shape for 3D collisions.
+		Separation ray shape resource for 3D physics.
 	</brief_description>
 	<description>
-		Separation ray shape for 3D collisions, which can be set into a [PhysicsBody3D] or [Area3D]. A ray is not really a collision body; instead, it tries to separate itself from whatever is touching its far endpoint. It's often useful for characters.
+		3D separation ray shape to be added as a [i]direct[/i] child of a [PhysicsBody3D] or [Area3D] using a [CollisionShape3D] node. A ray is not really a collision body; instead, it tries to separate itself from whatever is touching its far endpoint. It's often useful for characters.
+		[b]Performance:[/b] Being a primitive collision shape, [SeparationRayShape3D] is fast to check collisions against.
 	</description>
 	<tutorials>
 	</tutorials>