Explain kinematicbody 'infinite_inertia' better

**Issue description:**
At the moment, the way infinite inertia is explained makes it seem like you can't push rigidbodies, but you actually can, if can_sleep is false. It just stops you from pushing them inside of walls.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/physics/using_kinematic_body_2d.html
https://docs.godotengine.org/en/stable/classes/class_kinematicbody2d.html#class-kinematicbody2d

Also the 3d versions