Fix typo in field name in example
Add a document describing the content of documents

This gives details of the formats and semantics of fields included in
documents.

I wrote this partly in an attempt to ensure that I understand the
semantics of this, and partly to get some good documentation around this
for others. There are still some parts of this I'm uncertain about, so
I've marked them with TODO.
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.
Explain that some formats expect different fields

Move the description of the  field near the top, explain that different
fields are used by the special gone and redirect formats, and add a TODO
for the redirect field to document whether it'll be valid in formats
other than redirect once that decision is made.
Replace remaining uses of guid with "content ID"
Merge pull request #43 from alphagov/document-document-formats

Add a document describing the content of documents