Facilitate handling a single document in the context of automation / workflows

People ramping up on Nuxeo have a hard time with automation in the context of workflows because chains always start with a list of documents (even if there is only one).
They usually tend to use a workaround using the restore document input operation + an expression, that we had to document due to a lot of people running into this.
This should not be the way to solve the problem, Studio should provide a proper solution.

Potential impacts
Studio may not be able to handle properly the input/output if we change it dynamically within the operation itself