Context variables accepted by action functions need to be documented

[seanh]

I was doing this:

context = {'model': base.model, 'session': base.model.Session,
                    'user': toolkit.c.user or toolkit.c.author,
                    'extras_as_string': True}
group_dict = logic.get_action('group_show')(context,
                    {'id': group_id})

in an extension and one of the group_dicts fields, one that uses convert_to/from_extras, was coming out with the wrong value. It took me ages to realise that I had to pass 'extras_as_string': True in the context. I don't think this or other context variables are documented anywhere.