docs(NODE-3481): deprecate unref (#2936)

Author: andymina

lib/core/connection/connection.js

@@ -190,6 +190,7 @@ class Connection extends EventEmitter {
    * Unref this connection
    * @method
    * @return {boolean}
+   * @deprecated This function is deprecated and will be removed in the next major version.
    */
   unref() {
     if (this.socket == null) {

lib/core/connection/pool.js

@@ -604,6 +604,7 @@ Pool.prototype.logout = function(dbName, callback) {
 /**
  * Unref the pool
  * @method
+ * @deprecated This function is deprecated and will be removed in the next major version.
  */
 Pool.prototype.unref = function() {
   // Get all the known connections

lib/core/sdam/topology.js

@@ -742,8 +742,11 @@ class Topology extends EventEmitter {
     return this.s.state === STATE_CLOSED;
   }
 
+  /**
+   * @deprecated This function is deprecated and will be removed in the next major version.
+   */
   unref() {
-    emitWarning('not implemented: `unref`');
+    emitWarning('`unref` is a noop and will be removed in the next major version');
   }
 
   // NOTE: There are many places in code where we explicitly check the last isMaster

lib/db.js

@@ -951,6 +951,7 @@ Db.prototype.indexInformation = function(name, options, callback) {
 /**
  * Unref all sockets
  * @method
+ * @deprecated This function is deprecated and will be removed in the next major version.
  */
 Db.prototype.unref = function() {
   this.s.topology.unref();