Improve clarity of some text

 - reword introductory paragraph.
 - explain that base_path is a unique identifier in the content store.
 - name the field holding the UUID as `content_id`.
 - explain why need_ids isn't tags.
Reword the explanation of tags.

Give an example of the tag structure, and base the explanation on that.

Also, specify that we return the "title" of the tagged items in the
retrieving context, since that's the current plan.