Both the current documentation and the Confluence 5.5 documentation say that you can "Restrict your search results to a particular space, or to your favourite spaces, site spaces or personal spaces."

This is incorrect.

You used to be able to to this in Confluence 5.1.

But somewhere between 5.1 and 5.5, Site Spaces was removed as an option.

Ditto for Personal Spaces ... but it's the ability to exclude Personal Spaces which I really miss (but that's a separate issue).