I had the same need of filter notes by "kind", specifically to see only notes added as bookmarks using the web clipper tool.
In the Grammar page (included in previous answers) I found that by typing "source:web.clip" in the search field I got what I wanted.
There is also options for filter notes added by mail, etc..
source:[string] - matches notes that came from an application or data source that matches the argument string. Not all notes will have a "source" attribute. Standard source attributes queries include:
Matches notes that were clipped