doc: add tls server.close() callback docs

Also, tests to confirm its existence.

doc/api/tls.markdown

@@ -602,11 +602,11 @@ when the server has been bound.
 See `net.Server` for more information.
 
 
-### server.close()
+### server.close([callback])
 
 Stops the server from accepting new connections. This function is
 asynchronous, the server is finally closed when the server emits a `'close'`
-event.
+event.  Optionally, you can pass a callback to listen for the `'close'` event.
 
 ### server.address()
 

test/parallel/test-tls-connect-simple.js

@@ -26,6 +26,8 @@ var fs = require('fs');
 
 var clientConnected = 0;
 var serverConnected = 0;
+var serverCloseCallbacks = 0;
+var serverCloseEvents = 0;
 
 var options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
@@ -34,7 +36,12 @@ var options = {
 
 var server = tls.Server(options, function(socket) {
   if (++serverConnected === 2) {
-    server.close();
+    server.close(function() {
+      ++serverCloseCallbacks;
+    });
+    server.on('close', function() {
+      ++serverCloseEvents;
+    });
   }
 });
 
@@ -60,4 +67,6 @@ server.listen(common.PORT, function() {
 process.on('exit', function() {
   assert.equal(clientConnected, 2);
   assert.equal(serverConnected, 2);
+  assert.equal(serverCloseCallbacks, 1);
+  assert.equal(serverCloseEvents, 1);
 });