Document main function. about janet-lang.org HOT 2 CLOSED

@crocket I think @uvtc documented main in #155. Perhaps this issue can be closed.

from janet-lang.org.

It looks to me like it can be triggered in a couple of places within cli-main in boot.janet:

• https://github.com/janet-lang/janet/blob/40ae2e812ff5e186ba578d441de9f3d2bebd93fb/src/boot/boot.janet#L3691
• https://github.com/janet-lang/janet/blob/40ae2e812ff5e186ba578d441de9f3d2bebd93fb/src/boot/boot.janet#L3704

run-main is currently defined earlier in boot.janet as:

(defn- run-main
  [env subargs arg]
  (if-let [entry (in env 'main)
           main (or (get entry :value) (in (get entry :ref) 0))]
    (let [thunk (compile [main ;subargs] env arg)]
      (if (function? thunk) (thunk) (error (thunk :error))))))

from janet-lang.org.