Improve and update documentation of registers

Improve and update documentation of registers

* lisp/register.el (register-use-preview)
(register--read-with-preview-function)
(register-preview-function, register-preview-default-1)
(register-preview-default, register--preview-function)
(register-preview-info, register-command-info)
(register-preview-forward-line, register-preview-next)
(register-preview-previous, set-register, register-type)
(register--type, register-preview, register-preview-1)
(register-preview-get-defaults, register-read-with-preview)
(register-read-with-preview-traditional)
(register-read-with-preview-fancy, register-preview-delay): Doc
fixes.

* doc/emacs/regs.texi (Registers): Describe the new preview modes.

* etc/NEWS: Move the registers entry to its correct place, and
document the modified behavior.

(Bug#66394)