add a clarifying comment to checking nan

Author: ferrine

tests/scalar/test_loop.py

@@ -215,6 +215,7 @@ def test_inner_composite(mode):
     fn16 = function([n_steps, x16], y16, mode=mode)
     out16 = fn16(n_steps=9, x16=np.array(4.73, dtype="float16"))
     assert out16.dtype == "float16"
+    # with this dtype overflow happens
     assert np.isnan(out16)