Zapier supports hundreds of web apps, not all of which have help documentation yet.
You can see a list of all the supported apps here.
Apps with the Beta label are newly available in Zapier. If you're using one of the Beta apps, you're on the cutting edge of what's possible in Zapier.
Note that some of the apps listed on the Zapbook are not yet supported. This text (and the Upcoming label) indicates an app is currently under consideration for future development: