Docs are for master, not 1.2.x

Given users are recommended to use 1.2.x as a stable release, and there looks to be a large shift in API and even core plugins, I think it makes sense for the documentation to be for 1.2.x. While the differences aren't very large at the moment, this is a problem that will get worse as master and 1.2.x diverge and may be a source of pain for people who need the docs.