Document v2 API as experimental

In the recent "Should v2 API be 'experimental'" mailing list thread we decided that we wanted to declare the v2 API as "experimental", both as a way to signal users about its evolving state, and to allow us to change it for the better in ways that otherwise wouldn't be possible (i.e. without concern for backcompat).

This ticket covers documenting this in the ref-guide.  Notices should probably go in the 9.0 upgrade notes, and on the v2-api.adoc page itself.