I don't know if there is any documentation on it, but the search index can't handle phrase search. Only words in pages are indexed, so a phrase makes no sense to the search index. What you can do to generate fewer hits is to change the SearchDataSource markup so it only searches for whole words, which should make it a bit less promiscuous. The next step is to require and exclude words (+ and -, respectively, in front of the word). If you need more than that you should look into integrating a different search engine like Site Seeker, Google (you can get a custom indexer from google) or MS Search Server.