init_facets: Add documentation
init_facets is not included in the default sphinx documentation but this
docstring should help people digging into cubicweb core. This could go in an
Advanced developer section of the book.
Schema of the data
------------------
First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: schema