Improve the explanatory comment at the top of check_consistent.py

tests/check_consistent.py

@@ -1,7 +1,11 @@
 #!/usr/bin/env python3
 
-# Symlinks are bad on Windows, so we cannot use them in typeshed.
-# This checks that certain files are duplicated exactly.
+# For various reasons we need the contents of certain files to be
+# duplicated in two places, for example stdlib/2and3/builtins.pyi and
+# stdlib/2/__builtin__.pyi must be identical.  In the past we used
+# symlinks but that doesn't always work on Windows, so now you must
+# manually update both files, and this test verifies that they are
+# identical.  The list below indicates which sets of files must match.
 
 import os
 import filecmp