Exploring :value-type choices

Have been looking at constructing a defcustom variable composed
of an alist and coded this.

(defcustom galaxiakos-alist
  '((name . "Pandu")
    (activation . dual))
  "A user-customisable association list.
Users can access this variable via `M-x customize-variable' and modify
the entries directly."
  :type '(alist :key-type symbol :value-type (choice string symbol))
  :group 'galaxiakos)

Scrutinising the reference manual, I could not identify the possibilities
for :value-type.