Issue #16655: Explain why Idle's test_calltips has 'fragile' tests of builtins.

I do not expect a problem in 2.7, but backported this anyway, 'in case'.

Lib/idlelib/idle_test/test_calltips.py

@@ -34,6 +34,13 @@ tc = TC()
 
 signature = ct.get_arg_text  # 2.7 and 3.x use different functions
 class Get_signatureTest(unittest.TestCase):
+    # The signature function must return a string, even if blank.
+    # Test a variety of objects to be sure that none cause it to raise
+    # (quite aside from getting as correct an answer as possible).
+    # The tests of builtins may break if the docstrings change,
+    # but a red buildbot is better than a user crash (as has happened).
+    # For a simple mismatch, change the expected output to the actual.
+
 
     def test_builtins(self):
         # 2.7 puts '()\n' where 3.x does not, other minor differences