Add note for some usages of Plane in APIs #111459
                
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Some old APIs that may expect a Vector4 instead require a Plane: this is pretty confusing especially with tangents as one could interpret the API as wanting a value directly representing the tangent plane when in reality the Plane type is just used to carry one Vector3 and one extra sign component.
This commit attempts to clear any potential confusion by adding a note to the APIs using Planes in such a way.