Setting tooltip-gud-tips-p directly has no effect

[Glenn Morris]

As the subject says, under both the latest CVS HEAD and RC branch, setting
tooltip-gud-tips-p to a non-nil value via setq has no effect. This variable
is defined in tooltip.el as follows:

(defcustom tooltip-gud-tips-p nil
  "*Non-nil means show tooltips in GUD sessions."
  :type 'boolean
  :tag "GUD"
  :set #'(lambda (symbol on)
           (setq tooltip-gud-tips-p on)
           (if on (tooltip-gud-tips-setup)))
  :group 'tooltip)

I think the doc string would benefit from an addition along the lines of
"Setting this variable directly does not take effect; use either M-x
customize or the function tooltip-gud-tips-activate", where
tooltip-gud-tips-activate would do something along the lines of

(setq tooltip-gud-tips-p t)
(tooltip-gud-tips-setup)

WDYT?