docs: Add a caveat about expandByObject/setFromObject

The comment is worded to not interfere too much with possible future
changes to the function's implementation.

docs/api/en/math/Box3.html

@@ -138,6 +138,7 @@
 
 		Expands the boundaries of this box to include [page:Object3D object] and its children,
 		accounting for the object's, and children's, world transforms.
+		The function may result in a larger box than strictly necessary.
 
 		</p>
 
@@ -279,6 +280,7 @@
 
 		Computes the world-axis-aligned bounding box of an [page:Object3D] (including its children),
 		accounting for the object's, and children's, world transforms.
+		The function may result in a larger box than strictly necessary.
 
 		</p>