[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 schema"""