Improve tab-completion Sphinx documentation

[tleonhardt]

Update the completion.rst file to include most, if not all, of the header to argparse_custom.py.

The best way to do this would be to reformat that module docstring so that it is valid reStructuredText so that the Sphinx AutoDoc automodule command will automatically import it.

If that proves too difficult or time consuming, then we could manually cut-and-paste and reformat to valid reStructuredText.

[tleonhardt]

#887 updated the tab completion examples

[kmvanbrunt]

I still plan on updating these documents, but they don't need to delay the 1.0 release