When you do a regular interest search on LiveJournal, by visiting http://www.livejournal.com/interests.bml or by using the search box present at the top right of some LiveJournal site schemes, communities that list the interest will appear in the search results. If there are fewer than 1000 results, all communities that list the interest you're searching for will be at the top of the search results page. If there are more than 1000 results, they will be mixed in with the rest of the results.
We can discuss what we want to put in in place of that [if anything], but I just want that out of the FAQ because people are asking about it, and Support is telling them that it's been disabled. And having incorrect information in the FAQs is generally not a Good Thing.
[for reference purposes, searching for comms by interest has been disabled until they can optimise the search method. see http://www.livejournal.com/community/lj_dev/553289.html for more details]