Add descriptions for Watson NLU outputs at bottom of Analyze_Text.ipynb

[frreiss]

The last four cells of Analyze_Text.ipynb show the DataFrame version of Watson NLU's output for entities, keywords, relations, and semantic roles.

We should add some Markdown text in between these cells to explain what kind of data is stored in each column of each DataFrame. Where applicable, we should explain in plain English what the Watson NLU output in one example row of the DataFrame means.