Generic6DOFJoint3D documentation should explain how to recreate other joints

[aaronfranke]

Your Godot version: Latest master

Issue description:

I noticed that the documentation for Generic6DOFJoint3D mentions that it can be used to implement a variety of joint types, but it does not mention how one could recreate PinJoint3D or the other joints using Generic6DOFJoint3D. This would be hugely helpful for understanding how the Generic6DOFJoint3D constraints work.

URL to the documentation page:

Generic6DOFJoint3D https://docs.godotengine.org/en/latest/classes/class_generic6dofjoint3d.html
and all that extend Joint3D https://docs.godotengine.org/en/latest/classes/class_joint3d.html