Mention that ShapeCast2D and ShapeCast3D stop scanning at the first instance of collision.

Godot v4.2.2

Clarify the meaning of "ShapeCast2D can detect multiple collision objects" to clarify that it stops scanning the first instance of collision, just like a RayCast2D, but will return multiple shapes if that first collision hits multiple objects simultaneously.

As of now, can be confused with returning ALL collisions in its scanning range, more like an Area2D, except with ordered collision info. This confusion is further amplified by the example of using it as a substitution to an Area2D included in the current version of the docs.

[ShapeCast2D docs](https://docs.godotengine.org/en/stable/classes/class_shapecast2d.html)

[ShapeCast3D docs](https://docs.godotengine.org/en/stable/classes/class_shapecast3d.html)