Add a docstring describing the program

Prefer a docstring to a header comment so we can use it as the
ArgumentParser description (formatted with 'nmbug-status --help').
Script readers still have it near the top of the file.  Since it's a
docstring, use PEP 257's summary-line-and-body format [1].

[1]: http://legacy.python.org/dev/peps/pep-0257/#multi-line-docstrings