Link 'anonymous' users to help section with explanation.

Requests submitted using the write API may have an anonymous user. Add a
section to the FAQ explaining this, and link to it from those requests.
Note this commit removes some 'h' calls that are no longer needed in
Rails 3. Contributes to https://github.com/mysociety/alaveteli/issues/1286
Merge branch 'feature/1286-explain-anonymous-requests' into rails-3-develop