Document define-symbol-macro macro

[jcubic]

It's part of Devin Wiki.

It was created as a helper to define the vector literals #, but it's not actually documented (on the website, it only has a docstring).

;; Example: Vector literal syntax
(define-symbol-macro SPLICE (vector-literal "#" . args)
  (if (not (or (pair? args) (eq? args '())))
      (throw (new Error "vector require pair"))
      (let ((v (list->array args)))
        (Object.freeze v)
        v)))

Related issues:

• Remove support for macros in syntax-extensions #499
• Consider passing function instead of symbol to set-special! #478