update docs

Author: Ughuuu

docs/documentation/joints.md

@@ -23,3 +23,5 @@ The multibody joints support IK target nodes. In order to use it, create a Rapie
 Now, the whole chain structure will follow the **ik_target**:
 
 ![ik](/img/joints/ik.png)
+
+When setting a multibody joint, the rigidbodies need to not collide with eachother and be massless.

docs/documentation/rigidbodies.md

@@ -11,6 +11,7 @@ Configuring rigid bodies can be difficult. This page describes some of the most
 In order to improve simulation stackability and stability in general, Rapier Extension Classes (eg. RapierRigidBody2D, ..) offer custom settings, such as:
 - **body_skin**: adds a small skin on top of the body, defualt is 0
 - **soft_ccd**: setting this to a small value of how much the object might overlap helps with reducing squishiness.
+- **massless**: sets mass to 0. Needs to be set when using multibody joints.
 
 ## Ghost Collisions