Since Ozzy is not available at the moment, I'll answer.
There is no direct documentation. All of the settings for forms are fully commented and those comments explain what each setting does.
The purchase option is there because some people need multiple licenses for multiple forums. To download your current license click on your name in the upper right side of the site and select "Download my add-ons".
Unless Ozzy has changed something, 2.1+ refers to 2.1 and anything higher than that. So, 2.2 would be included provided it does not require a complete rewrite as was needed from 1.5 to 2.0 and again when enough changes were made for 2.1 to make it incompatible with 2.0. It boils down to when XenForo declares a XenForo version as End of Life, all add-ons here for a specific XenForo version will most likely follow and be declared End of Life. But again, that's up to Ozzy.
The XenForo information needs to be updated. 2.0 would not be included in the Lifetime support because it is already End of Life. Ozzy needs to do that, I can't access that any more since Ozzy now owns the rights to and supports the add-ons.
https://snogssite.com/threads/xf1-and-xf-2-0-add-ons-end-of-life.2807/https://snogssite.com/pages/lifetime/
Also so far as I know, there is no XenForo 2.2 at the moment.