[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.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile