Apple doesn't always sweat the details.
OS X installs documentation into both Library/Documentation and Users/Library/Documentation. The former location, for example, includes my iMac Users Guide, "Welcome to Snow Leopard" and some help files.
Problem is, Spotlight doesn't index Library/Documentation. It only indexes items within the Users folder. So a Spotlight search won't find those documents.
This isn't new, but it's a detail Apple hasn't bothered with.