Add docs to the sample-script explaining why it has an argv check.

python/subunit/tests/sample-script.py

@@ -1,6 +1,9 @@
 #!/usr/bin/env python
 import sys
 if len(sys.argv) == 2:
+    # subunit.tests.test_test_protocol.TestExecTestCase.test_sample_method_args 
+    # uses this code path to be sure that the arguments were passed to
+    # sample-script.py
     print "test fail"
     print "error fail"
     sys.exit(0)