register.el doc fixes

register.el doc fixes

* lisp/register.el (register-read-with-preview, point-to-register)
(window-configuration-to-register, frame-configuration-to-register)
(jump-to-register, number-to-register, view-register, insert-register)
(copy-to-register, append-to-register, prepend-to-register)
(copy-rectangle-to-register): Doc fixes.