docs(readConcern): update all possible readConcern levels

Part of NODE-1628

lib/db.js

@@ -113,7 +113,7 @@ const legalOptionNames = [
  * @param {object} [options.pkFactory] A primary key factory object for generation of custom _id keys.
  * @param {object} [options.promiseLibrary] A Promise library class the application wishes to use such as Bluebird, must be ES6 compatible
  * @param {object} [options.readConcern] Specify a read concern for the collection. (only MongoDB 3.2 or higher supported)
- * @param {object} [options.readConcern.level='local'] Specify a read concern level for the collection operations, one of [local|majority]. (only MongoDB 3.2 or higher supported)
+ * @param {ReadConcernLevel} [options.readConcern.level='local'] Specify a read concern level for the collection operations (only MongoDB 3.2 or higher supported)
  * @property {(Server|ReplSet|Mongos)} serverConfig Get the current db topology.
  * @property {number} bufferMaxEntries Current bufferMaxEntries value for the database
  * @property {string} databaseName The name of the database this instance represents.
@@ -355,7 +355,7 @@ const collectionKeys = [
  * @param {boolean} [options.serializeFunctions=false] Serialize functions on any object.
  * @param {boolean} [options.strict=false] Returns an error if the collection does not exist
  * @param {object} [options.readConcern] Specify a read concern for the collection. (only MongoDB 3.2 or higher supported)
- * @param {object} [options.readConcern.level='local'] Specify a read concern level for the collection operations, one of [local|majority]. (only MongoDB 3.2 or higher supported)
+ * @param {ReadConcernLevel} [options.readConcern.level='local'] Specify a read concern level for the collection operations (only MongoDB 3.2 or higher supported)
  * @param {Db~collectionResultCallback} [callback] The collection result callback
  * @return {Collection} return the new Collection instance if not in strict mode
  */

lib/mongo_client.js

@@ -46,6 +46,12 @@ const closeOperation = require('./operations/mongo_client_ops').closeOperation;
  * });
  */
 
+/**
+ * A string specifying the level of a ReadConcern
+ * @typedef {'local'|'available'|'majority'|'linearizable'|'snapshot'} ReadConcernLevel
+ * @see https://docs.mongodb.com/manual/reference/read-concern/index.html#read-concern-levels
+ */
+
 /**
  * Creates a new MongoClient instance
  * @class
@@ -88,7 +94,7 @@ const closeOperation = require('./operations/mongo_client_ops').closeOperation;
  * @param {object} [options.pkFactory] A primary key factory object for generation of custom _id keys
  * @param {object} [options.promiseLibrary] A Promise library class the application wishes to use such as Bluebird, must be ES6 compatible
  * @param {object} [options.readConcern] Specify a read concern for the collection (only MongoDB 3.2 or higher supported)
- * @param {string} [options.readConcern.level='local'] Specify a read concern level for the collection operations, one of [local|majority]. (only MongoDB 3.2 or higher supported)
+ * @param {ReadConcernLevel} [options.readConcern.level='local'] Specify a read concern level for the collection operations (only MongoDB 3.2 or higher supported)
  * @param {number} [options.maxStalenessSeconds=undefined] The max staleness to secondary reads (values under 10 seconds cannot be guaranteed)
  * @param {string} [options.loggerLevel=undefined] The logging level (error/warn/info/debug)
  * @param {object} [options.logger=undefined] Custom logger object
@@ -300,7 +306,7 @@ MongoClient.prototype.isConnected = function(options) {
  * @param {object} [options.pkFactory] A primary key factory object for generation of custom _id keys
  * @param {object} [options.promiseLibrary] A Promise library class the application wishes to use such as Bluebird, must be ES6 compatible
  * @param {object} [options.readConcern] Specify a read concern for the collection (only MongoDB 3.2 or higher supported)
- * @param {string} [options.readConcern.level='local'] Specify a read concern level for the collection operations, one of [local|majority]. (only MongoDB 3.2 or higher supported)
+ * @param {ReadConcernLevel} [options.readConcern.level='local'] Specify a read concern level for the collection operations (only MongoDB 3.2 or higher supported)
  * @param {number} [options.maxStalenessSeconds=undefined] The max staleness to secondary reads (values under 10 seconds cannot be guaranteed)
  * @param {string} [options.loggerLevel=undefined] The logging level (error/warn/info/debug)
  * @param {object} [options.logger=undefined] Custom logger object