Update doc for Matrix4

Author: FlorentMasson

docs/api/en/math/Matrix4.html

@@ -88,7 +88,7 @@ <h2>Extracting position, rotation and scale</h2>
 					[page:Vector3.setFromMatrixScale]: can be used to extract the scale component.
 				</li>
 				<li>
-					[page:Quaternion.setFromRotationMatrix], [page:Euler.setFromRotationMatrix] or [page:.extractRotation extractRotation] can be used to extract the rotation component.
+					[page:Quaternion.setFromRotationMatrix], [page:Euler.setFromRotationMatrix] or [page:.extractRotation extractRotation] can be used to extract the rotation component from a pure (unscaled) matrix.
 				</li>
 				<li>
 					[page:.decompose decompose] can be used to extract position, rotation and scale all at once.