improve docs to show signatures type

lib/script/script.js

@@ -591,7 +591,7 @@ Script.buildMultisigOut = function(publicKeys, threshold, opts) {
  *
  * @param {PublicKey[]} pubkeys list of all public keys controlling the output
  * @param {number} threshold amount of required signatures to spend the output
- * @param {Array} signatures signatures to append to the script
+ * @param {Array} signatures and array of signature buffers to append to the script
  * @param {Object=} opts
  * @param {boolean=} opts.noSorting don't sort the given public keys before creating the script (false by default)
  * @param {Script=} opts.cachedMultisig don't recalculate the redeemScript