RayCast2D and RayCast3D: warn to check is_colliding before get_collision_point and get_collision_normal

[rburing]

Adds warnings to check is_colliding first, in the documentation of get_collision_point and get_collision_normal.

The warning is phrased in the positive, making no guarantees about what happens if is_colliding returns false. In that case, if using Godot Physics, the vectors will be zero if the ray never collided, or they will contain the values from the previous time when there was a collision, but I think this implementation detail is not worth documenting. (It may be worth adding an error for that case in the future.)

Also moved some adjacent information from a note to the main text, since the main text was just incomplete without it.

• Closes RayCast3D doesn't update collision point when calling force_raycast_update() #84642

[Mickeon]

Also needs to be rebased. The part about global coordinates is handled more nicely in this PR, by being part of the same paragraph instead of a separate note.

[akien-mga]

Thanks!