On the docs page, in the Index Filters section (http://docs.mongodb.org/master/core/query-plans/#index-filters), there is an "Index Filter" link.

It's in paragraph 2:

"When an index filter exists for the query shape..."

The link just takes you up an inch or two on a large monitor.

This is a bummer because users may click on the link and get annoyed that they move their screen not at all, or almost not at all.