Only three branches are maintained at any one time; these are master, the current, and the former Client Desktop release series. Any change to the documentation is made in a branch based off of master. Once the branch's PR is approved and merged, the PR is backported to the branch for the current Desktop release and the former release but only if it applies to it.
When a new ownCloud major or minor Desktop version is released, a new branch is created to track the changes for that release. The branch for the oldest release is frozen, taken off the active maintained branch list and is no longer maintained.