For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
PhysicsBody now has methods move_and_collide/test_move and needed
properties for these methods: safe margin, locked axes (3D only).
Moved collision_exceptions from StaticBody to PhysicsBody for 3D
(same as 2D, and conforms to documentation).
RigidBody doesn't have test_motion method anymore, it's now redundant
with PhysicsBody.test_move.
Helps a lot with soft bodies and generally useful to avoid shapes to go
through the ground in certain cases.
Added an option in ConcavePolygonShape to re-enable backface collision
on specific bodies if needed.
