Add documentation for accessing dictionary parameter values

[nielsdejong]

We should document explicitly this behaviour:

http://neodash.graphapp.io/?share&type=file&id=https%3A%2F%2Fgist.githubusercontent.com%2Fnielsdejong%2F72aa0a1af7f5d12b6c23d3e09e1c1ccd%2Fraw%2F308f966987fedbea13bcb49288193b7aa8e6607a%2Fmovies-action-markdown.json&dashboardDatabase=neo4j&credentials=neo4j%2Bs%3A%2F%2Fmovies%3Amovies%40movies%3Ademo.neo4jlabs.com%3A7687

Setting a dashboard parameter to a dictionary, then accessing a property inside a markdown block)