Only three branches are maintained at any one time; these are master the current, and the former Server release series. Any change to the documentation is made in a branch based off of master with only a view exceptions.
Once the branch's PR is approved and merged, the PR is backported to the branch for the current Server release and the former release, but only if it applies to it.
When a new ownCloud major or minor version is released, a new branch is created to track the changes for that release, and the branch for the previous release is no longer maintained. That said, changes for patches and bugfixes to some earlier versions are backported. Also see the Maintenance and Release Schedule