add-variable-watcher usage in lisp/frame.el

[Robert Pluim]

lisp/frame.el has:

(mapc (lambda (var)
        (add-variable-watcher var (symbol-function 'set-buffer-redisplay)))

which causes (describe-variable 'truncate-lines) to produce output like

  Automatically becomes permanently buffer-local when set.
  Calls these functions when changed: (#<subr set-buffer-redisplay>)

Note the "#<subr... there.

Not using symbol-function makes this output (set-buffer-redisplay)
instead, which I think looks better, Iʼm wondering why symbol-function
was used?

Thanks

Robert