Add an explanation comment.

Author: serhiy-storchaka

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -1949,6 +1949,13 @@ class CTestModule(TestModule):
 
 class MiscTests(unittest.TestCase):
     def test_pydatetime(self):
+        # Test that zoneinfo works if the C implementation of datetime
+        # is not available and the Python implementation of datetime is used.
+        # The Python implementation of zoneinfo should be used in thet case.
+        #
+        # Run the test in a subprocess, as importing _zoneinfo with
+        # _datettime disabled causes crash in the previously imported
+        # _zoneinfo.
         assert_python_ok('-c', '''if 1:
             import sys
             sys.modules['_datetime'] = None