Minor docstring inaccuracies about featuretools

[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?