apidoc extension docs don't list all available options (at least `--module-first` missing)

[pekkaklarck]

I wanted to have module contents before submodules but didn't find anything related when looking at docs. Looking at the source I noticed that --module-first does exactly what I wanted.

[tk0miya]

MEMO: related with #2250

[tk0miya]

Now the reference is updated.
http://www.sphinx-doc.org/en/master/man/sphinx-apidoc.html#cmdoption-sphinx-apidoc-m

Thank you for reporting!