Insights responses now describe their fields

The five Insights endpoints now describe their response fields. A few details that weren't documented before:

• sentiment_score is net sentiment: positive minus negative, as a percentage of the total (-100 to 100).
• reviewed_at is a Unix timestamp in milliseconds.
• On Reviews per topic and Reviews per product, paging.total and paging.page_count can be 0 even when reviews are returned, so don't rely on them to stop paging.