[book] some more documentation and cleanups
* merged existing facets documentation with the one I've just written and
put almost everything within the code
* added a note about __depends__ and __recommends__ and about the recommends
semantic (extracted from a post on the ml)
* added a note about write security checking (extracted from a post on the ml)
* fixed some dumb sphinx errors
The property mecanism
---------------------
.. XXX CWProperty and co
Property API
~~~~~~~~~~~~
.. XXX feed me
Registering and using your own property
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. XXX feed me