Search API doesn't return featured snaps as documented

The snap search API at https://api.snapcraft.io/api/v1/snaps/search doesn't work as documented.

The documentation (at https://search.apps.ubuntu.com/docs/#id5) says:

> In the special case that section param is present but empty, a search for globally featured will be performed:

I took this to mean that:

$ curl "https://api.snapcraft.io/api/v1/snaps/search?section="

Should return the same list of featured snaps as:

$ snap find

Instead, it returns a much longer list.

To get the actual list of featured snaps, as returned by `snap find`, I need to call:

$ curl "https://api.snapcraft.io/api/v1/snaps/search?confinement=strict,classic&q=&section=featured"

This does not match the documentation.