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)
1 job for master in 4 minutes and 57 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Normal
failed #78718
test-lisp-inotify

00:04:57

 
Name Stage Failure
failed
test-lisp-inotify Normal
Uploading artifacts for failed job
Uploading artifacts...
test-lisp-inotify-7058988f/: found 119 matching files and directories

Uploading artifacts as "archive" to coordinator... 201 Created
id=78718 responseStatus=201 Created token=L74hUrEw
Uploading artifacts...
test-lisp-inotify-7058988f/junit-test-report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... 201 Created
id=78718 responseStatus=201 Created token=L74hUrEw
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 2