bug#3526: 23.0.94; `list-character-sets' display

Thanks for this info. Maybe we could have a link from the `list-character-sets'
display to the Info node that explains this?

And maybe we could include a one-liner that explains (most of) what it means?
E.g., as the doc string says: "used only as a parent or a subset of some other
charset"

These things would help, I think: one-line reminder, and link to obtain more
info. I expect that many users are not too familiar with these things.