What exactly is the 'user' on a post_timeline?
I am assuming that it is the user responsible for the action, if not the user, but I cannot be sure as the JSON structure used to document the results does not lend itself to documenting nested objects or arrays.
It is also not clear that it is 'optional'. Again, I attribute this to the limitations of the documentation structure.
Perhaps the results structure could be parsed and rendered as HTML as the parameters are and this information, description and optional, could be provided to us.