Minor docstring inaccuracies

[Seth-Rothschild]

I came accross these two issues:

1. In entity_from_dataframe it says secondary_time_index is a string, when it should be a dictionary.
2. In normalize_entity there's an undocumented kwarg variable_types. I'm thinking of using the branch to fix them to also standardize the notation in those two docstrings for lists, dicts and optional arguments to

   list[str]
   (dict[str->list[str]], optional)

[Seth-Rothschild]

It's worth noting that other pages follow slightly different styles as well. make_agg_primitive uses distinct, possibly better notation which states list[Variable] with special typesetting for Variable to indicate it's a featuretools class. Maybe it's better to spec to that and try to standardize all of the long docstrings at once?

I'm thinking:

1. entity_fromdataframe`
2. normalize_entity
3. make_agg_primitive and make_trans_primitive
4. dfs
5. calculate_feature_matrix

Am I missing any other big ones?