Improve Shape2D's `custom_solver_bias` description

[Calinou]

This backports the description from master. See https://github.com/godotengine/godot/pull/51121#issuecomment-1118008465.

[akien-mga]

The docs refer to a missing property.

[Calinou]

The docs refer to a missing property.

It looks like the default bias isn't adjustable in 3.x. It's hardcoded to 0.3 in both 2D and 3D:

https://github.com/godotengine/godot/blob/42fe8ab084fbf16d3dc250f26ff7744db928bd68/servers/physics_2d/body_pair_2d_sw.cpp#L338

https://github.com/godotengine/godot/blob/42fe8ab084fbf16d3dc250f26ff7744db928bd68/servers/physics/body_pair_sw.cpp#L263

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.1.