The HHVM blog has a post today announcing some updates they've made around the documentation for the project and the release of the "next generation" of their documentation at http://docs.hhvm.com/.
Back in August, we announced that we are going full force in revamping user documentation. We sent out a public survey to gauge the standing on the existing documentation at the time. We had 160 responses to the survey. Those results served as both validation and a guide to our approach with the new documentation.
The survey showed some interesting results including that the existing documentation could use improvement, better content in certain sections and poor examples in some places. In order to help this they worked hard to revamp the documentation and created a new GitHub repository for the docs and allows developers to pull it down locally and contribute back content/corrections as they might catch them. They also lay out the new documentation structure, breaking it up into Hack, API and HHVM sections. Finally, they talk about the technology behind the site including the runnable code examples, how they're generated and what the build process looks like.