Hello!

This is a follow-up to a comment I made on bug#73827. The register-list
package only supports frames, but not framesets. The below patch is not
particularly elegant, because framesets are saved as registers as type
frameset-register, so the frameset-p predicate does not apply here. This
is a pretty simple change and if there is a better way, I am obviously
open to it.

Also note that I did not remove support for the frame configurations. I
instead kept type "F" for both and updated the documentation. Because
neither the values of frames nor framesets can be edited, this did not
present an issue in testing.