docs: document the fature flag to enable the unified topology

Author: mbroadst

lib/mongo_client.js

@@ -110,6 +110,7 @@ const closeOperation = require('./operations/mongo_client_ops').closeOperation;
  * @param {boolean} [options.monitorCommands=false] Enable command monitoring for this client
  * @param {number} [options.minSize] If present, the connection pool will be initialized with minSize connections, and will never dip below minSize connections
  * @param {boolean} [options.useNewUrlParser=false] Determines whether or not to use the new url parser. Enables the new, spec-compliant, url parser shipped in the core driver. This url parser fixes a number of problems with the original parser, and aims to outright replace that parser in the near future.
+ * @param {boolean} [options.useUnifiedTopology] Enables the new unified topology layer
  * @param {MongoClient~connectCallback} [callback] The command result callback
  * @return {MongoClient} a MongoClient instance
  */