[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.
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s views/forms/actions/components for web ui"""