Issue #23738: Document and test actual keyword parameter names

Also fix signature because os.utime(..., ns=None) is not allowed.

Lib/test/test_popen.py

@@ -57,6 +57,10 @@ class PopenTest(unittest.TestCase):
         with os.popen("echo hello") as f:
             self.assertEqual(list(f), ["hello\n"])
 
+    def test_keywords(self):
+        with os.popen(cmd="exit 0", mode="w", buffering=-1):
+            pass
+
 def test_main():
     support.run_unittest(PopenTest)