docs(aggregate): add note explaining near

Author: vkarpov15

lib/aggregate.js

@@ -392,6 +392,7 @@ Aggregate.prototype.project = function(arg) {
  * @memberOf Aggregate
  * @instance
  * @param {Object} arg
+ * @param {Object|Array<Number>} arg.near GeoJSON point or coordinates array
  * @return {Aggregate}
  * @api public
  */