Added missing partial index param in the docs

Author: cuchi

lib/collection.js

@@ -1576,6 +1576,7 @@ define.classMethod('isCapped', {callback: true, promise:true});
  * @param {number} [options.v=null] Specify the format version of the indexes.
  * @param {number} [options.expireAfterSeconds=null] Allows you to expire data on indexes applied to a data (MongoDB 2.2 or higher)
  * @param {number} [options.name=null] Override the autogenerated index name (useful if the resulting name is larger than 128 bytes)
+ * @param {object} [options.partialFilterExpression=null] Creates a partial index based on the given filter object (MongoDB 3.2 or higher)
  * @param {object} [options.collation=null] Specify collation (MongoDB 3.4 or higher) settings for update operation (see 3.4 documentation for available fields).
  * @param {Collection~resultCallback} [callback] The command result callback
  * @return {Promise} returns Promise if no callback passed