add comment as per review

Author: kmuehlbauer

xarray/tests/test_backends.py

@@ -630,6 +630,8 @@ def test_roundtrip_boolean_dtype(self) -> None:
         with self.roundtrip(original) as actual:
             assert_identical(original, actual)
             assert actual["x"].dtype == "bool"
+            # this checks for preserving dtype during second roundtrip
+            # see https://github.com/pydata/xarray/issues/7652#issuecomment-1476956975
             with self.roundtrip(actual) as actual2:
                 assert_identical(original, actual2)
                 assert actual2["x"].dtype == "bool"