So far they have only been talked about in source code
(HistoryRegister), not in documentation.
Let's give them an official name so users can find them better;
"prompt history register" seems better than "history register" since
the former gives more context. OTOH, in future other registers (like @)
could grow into history registers, so I'm not sure.
State that the history registers are only changed by _interactive_
prompts; that's not quite true yet for user modes but I'll push a
separate fix for that.
c335712e4e