"Choose a ceramic capacitor that has an impedance null (self resonance) at the ring frequency or higher."
So, in theory, a low pass filter's performance gets better and better as you go up in frequency.
Unfortunately, that's only true for "ideal" components, which don't exist in the real world.
If you search for "ceramic capacitor self resonance" as a google image search, you'll see results like this:
The straight lines in that graph represent what the performance would be for ideal components, and the lines which dip and then go back up are the kind of performance you can get with real-world components.
The lessons here are that ceramic caps have a self-resonant frequency, and right at that frequency their performance gets a little bit better (the dip), but above that frequency their performance starts getting worse and worse (the lines start curving back up). Also, you can see from the graph that difference sized capacitors have different self resonant frequencies (and the larger the cap, the lower the frequency).
In EEVBlog #343 (
) Dave tests out an LC filter using a spectrum analyzer and tracking generator. There, he demonstrates this effect. The filter works great up until about 75 MHz, then starts getting less effective as the frequency gets higher.
So the reason they say to pick a cap which has a resonant frequency right where you want to filter is that you get a slight boost in performance right at that frequency. But "don't best be the enemy of good" (don't let not understanding that slow you down. Order some 1uF and 0.1uF ceramic caps and try them out!).
(I should have also mentioned that if you are going to try an LC filter, you'll find that small toroid inductors from china on ebay are ridiculously cheap, especially compared to more trustworthy parts found on digikey. Because your switcher is only $6, I'd recommend buying a handful of 100uH toroid inductors rated for a few amps from ebay and fool around with them (and then follow them with a 317). e.g.
http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=toroid+inductor+100uh&rt=nc&LH_BIN=1 )