doc registers: document prompt history registers

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.

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