examples: comment the usage of process.nextTick

closes #2903
closes #2908

examples/error/index.js

@@ -31,6 +31,9 @@ app.get('/', function(req, res){
 
 app.get('/next', function(req, res, next){
   // We can also pass exceptions to next()
+  // The reason for process.nextTick() is to show that
+  // next() can be called inside an async operation,
+  // in real life it can be a DB read or HTTP request.
   process.nextTick(function(){
     next(new Error('oh no!'));
   });

examples/view-locals/user.js

@@ -9,6 +9,9 @@ function User(name, age, species) {
 }
 
 User.all = function(fn){
+  // process.nextTick makes sure this function API
+  // behaves in an asynchronous manner, like if it
+  // was a real DB query to read all users.
   process.nextTick(function(){
     fn(null, users);
   });