[facets] FacetFilterMixIn: add documention for class and generate_form
There is also a small refactoring to make the code clearer. An intermediate
variable is introduced and the call is reindented to make it clearer. No logic
change.
include *.py
include */*.py
recursive-include data *.gif *.png *.ico *.css *.js
recursive-include i18n *.po
recursive-include wdoc *