docs(ngModel): rename $asyncValidators error to nopromise and add missing error page

docs/content/error/ngModel/nopromise.ngdoc

@@ -0,0 +1,28 @@
+@ngdoc error
+@name ngModel:nopromise
+@fullName No promise
+@description
+
+The return value of an async validator, must always be a promise. If you want to return a
+non-promise value, you can convert it to a promise using {@link ng.$q#resolve `$q.resolve()`} or
+{@link ng.$q#reject `$q.reject()`}.
+
+Example:
+
+```
+.directive('asyncValidator', function($q) {
+  return {
+    require: 'ngModel',
+    link: function(scope, elem, attrs, ngModel) {
+      ngModel.$asyncValidators.myAsyncValidation = function(modelValue, viewValue) {
+        if (/* I don't need to hit the backend API */) {
+          return $q.resolve();     // to mark as valid or
+          // return $q.reject();   // to mark as invalid
+        } else {
+          // ...send a request to the backend and return a promise
+        }
+      }
+    }
+  };
+})
+```

src/ng/directive/ngModel.js

@@ -638,7 +638,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
       forEach(ctrl.$asyncValidators, function(validator, name) {
         var promise = validator(modelValue, viewValue);
         if (!isPromiseLike(promise)) {
-          throw ngModelMinErr("$asyncValidators",
+          throw ngModelMinErr('nopromise',
             "Expected asynchronous validator to return a promise but got '{0}' instead.", promise);
         }
         setValidity(name, undefined);

test/ng/directive/ngModelSpec.js

@@ -906,7 +906,7 @@ describe('ngModel', function() {
 
         expect(function() {
           scope.$apply('value = "123"');
-        }).toThrowMinErr("ngModel", "$asyncValidators",
+        }).toThrowMinErr("ngModel", "nopromise",
           "Expected asynchronous validator to return a promise but got 'true' instead.");
       }));