[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
new RelationAttributeFacet + other fixes and cleanup + facets test
* added documentation to Relation[Attribute]Facet and AttributeFacet explaining
how to configure them
* fixed behaviour of sorting for various combinaison of attribute values
[migration] remove bad test when constructing attributes to copy in rename_entity_type: the old entity type is not in the fschema due to schema deserialization trick