In reply to brad.

It goes without saying that all of this information is coming directly from the WordPress source code. We're literally parsing the information out into a code reference.

Most modern IDE's are already privy to most of this information, though some of the specialized things, like hook documentation, may be a little ways off. I believe PhpStorm 8 will be shipping with some integrated WordPress functionality.

All that said, both the WP-Parser plugin and theme are open source.

You can find and contribute to WP-Parser here: https://github.com/rmccue/WP-Parser
And you can find a basic copy of the theme here: https://github.com/rarst/wporg-developer , though development has moved to meta.trac.wordpress.org.