Document physics material (#4817)

(cherry picked from commit b999f71b091e1b833b6202c3d3af049f12bb480d)

tutorials/physics/physics_introduction.rst

@@ -51,6 +51,13 @@ The other three bodies extend :ref:`PhysicsBody2D <class_PhysicsBody2D>`:
     A body that provides collision detection, but no physics. All movement and
     collision response must be implemented in code.
 
+Physics material
+~~~~~~~~~~~~~~~~
+
+Static bodies and rigid bodies can be configured to use a :ref:`physics material
+<class_PhysicsMaterial>`. This allows adjusting the friction and bounce of an object,
+and set if it's absorbent and/or rough.
+
 Collision shapes
 ~~~~~~~~~~~~~~~~
 

tutorials/physics/rigid_body.rst

@@ -12,7 +12,11 @@ In order to define the shape of the body, it must have one or more :ref:`Shape <
 How to control a rigid body
 ---------------------------
 
-A rigid body's behavior can be altered by setting its properties, such as friction, mass, bounce, etc. These properties can be set in the Inspector or via code. See :ref:`RigidBody <class_RigidBody>` for the full list of properties and their effects.
+A rigid body's behavior can be altered by setting its properties, such as mass and weight.
+A physics material needs to be added to the rigid body to adjust its friction and bounce,
+and set if it's absorbent and/or rough. These properties can be set in the Inspector or via code.
+See :ref:`RigidBody <class_RigidBody>` and :ref:`PhysicsMaterial <class_PhysicsMaterial>` for
+the full list of properties and their effects.
 
 There are several ways to control a rigid body's movement, depending on your desired application.