MAINT/DOC: spatial: Document and test the double cover property for R…

…otation quaternions (scipy#18955)

* Document and test the double cover property for quaternions

scipy/spatial/transform/tests/test_rotation.py

@@ -926,7 +926,7 @@ def test_approx_equal():
 
 def test_approx_equal_single_rotation():
     # also tests passing single argument to approx_equal
-    p = Rotation.from_rotvec([0, 0, 1e-9])  # less than default atol 1e-8
+    p = Rotation.from_rotvec([0, 0, 1e-9])  # less than default atol of 1e-8
     q = Rotation.from_quat(np.eye(4))
     assert p.approx_equal(q[3])
     assert not p.approx_equal(q[0])