Update jsdoc for PropertyEffects.splice

Array.prototype.splice has its "deleteCount" parameter optional. When omitted, all
items of the arrays are removed. This works with a polymer element
"splice" method, but the typing does not correspond and TypeScript
prevents it.

Fixes Polymer#5366

lib/mixins/property-effects.js

@@ -1873,7 +1873,7 @@ export const PropertyEffects = dedupingMixin(superClass => {
      *
      * @param {string | !Array<string|number>} path Path to array.
      * @param {number} start Index from which to start removing/inserting.
-     * @param {number} deleteCount Number of items to remove.
+     * @param {number=} deleteCount Number of items to remove.
      * @param {...*} items Items to insert into array.
      * @return {Array} Array of removed items.
      * @public