[document workflow] Branch arguments should contain source and target

When requesting hints for a workflow you must specify for which branch you want the hints (if none is provided it defaults to master). The hint for the branch action should however only be true if requested for master. But since the hints are validated before actually executing a workflow action you would never be able to create a branch other than master. So, for branching we actually need two branchIds: the source and the target branchId. The branch hint should then be evaluated for the source branchId and the execution of the branch action requires sourceBranchId, targetBranchId and targetBranchName.