test: update the docstring of 'test-devel-warnings.t' extension

We are testing more than just locks now.

tests/test-devel-warnings.t

@@ -1,6 +1,6 b''
 
   $ cat << EOF > buggylocking.py
-  > """A small extension that acquire locks in the wrong order
+  > """A small extension that tests our developer warnings
   > """
   > 
   > from mercurial import cmdutil, repair, revset