nmbug-status: Shift the script description into a docstring

nmbug-status: Shift the script description into a docstring

This way we can use it as the ArgumentParser description (formatted
with 'nmbug-status --help') and script readers still have it near the
top of the file.  I rephrased things a bit to match PEP 257's
summary-line-and-body format [1] while elaborating on the "if
available" Gmane views.

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