docs(guide/migration): add info for 1.4 (ng)Pattern BC

Breaking change was introduced in commit 0e001084ff.
This content being included in the migration guide is taken from the commit message of commit 0e001084ff.

Closes #15758
Closes #15765

docs/content/guide/migration.ngdoc

@@ -1455,7 +1455,7 @@ For more info on the topic, you can take a look at this
 ## Migrating from 1.3 to 1.4
 
 AngularJS 1.4 fixes major animation issues and introduces a new API for `ngCookies`. Further, there
-are changes to `ngMessages`, `$compile`, `ngRepeat`, `ngOptions `and some fixes to core filters:
+are changes to `ngMessages`, `$compile`, `ngRepeat`, `ngOptions`, `ngPattern`, `pattern` and some fixes to core filters:
 `limitTo` and `filter`.
 
 The reason for the ngAnimate refactor was to fix timing issues and to expose new APIs to allow
@@ -1469,9 +1469,9 @@ to render error messages with ngMessages that are listed with a directive such a
 involves pulling error message data from a server and then displaying that data via the mechanics of ngMessages. Be
 sure to read the breaking change involved with `ngMessagesInclude` to upgrade your template code.
 
-Other changes, such as the ordering of elements with ngRepeat and ngOptions, may also affect the behavior of your
-application. And be sure to also read up on the changes to `$cookies`. The migration jump from 1.3 to 1.4 should be
-relatively straightforward otherwise.
+Other changes, such as the ordering of elements with ngRepeat and ngOptions and the way ngPattern and pattern directives 
+validate the regex, may also affect the behavior of your application. And be sure to also read up on the changes to `$cookies`. 
+The migration jump from 1.3 to 1.4 should be relatively straightforward otherwise.
 
 
 
@@ -1575,7 +1575,7 @@ class based animations (animations triggered via ngClass) in order to ensure tha
 
 
 
-### Forms (`ngMessages`, `ngOptions`, `select`)
+### Forms (`ngMessages`, `ngOptions`, `select`, `ngPattern` and `pattern`)
 
 #### ngMessages
 The ngMessages module has also been subject to an internal refactor to allow it to be more flexible
@@ -1683,6 +1683,79 @@ ngModelCtrl.$formatters.push(function(value) {
 });
 ```
 
+#### ngPattern and pattern
+
+Due to [0e001084](https://github.com/angular/angular.js/commit/0e001084ffff8674efad289d37cb16cc4e46b50a),
+The `ngPattern` and `pattern` directives will validate the regex
+against the `$viewValue` of `ngModel`, i.e. the value of the model
+before the $parsers are applied. Previously, the `$modelValue`
+(the result of the $parsers) was validated.
+
+This fixes issues where `input[date]` and `input[number]` cannot
+be validated because the `$viewValue` string is parsed into
+`Date` and `Number` respectively (starting with Angular 1.3).
+It also brings the directives in line with HTML5 constraint
+validation, which validates against the input value.
+
+This change is unlikely to cause applications to fail, because even
+in Angular 1.2, the value that was validated by pattern could have
+been manipulated by the $parsers, as all validation was done
+inside this pipeline.
+
+If you rely on the pattern being validated against the `$modelValue`,
+you must create your own validator directive that overwrites
+the built-in pattern validator:
+
+```
+.directive('patternModelOverwrite', function patternModelOverwriteDirective() {
+  return {
+    restrict: 'A',
+    require: '?ngModel',
+    priority: 1,
+    compile: function() {
+      var regexp, patternExp;
+
+      return {
+        pre: function(scope, elm, attr, ctrl) {
+          if (!ctrl) return;
+
+          attr.$observe('pattern', function(regex) {
+            /**
+             * The built-in directive will call our overwritten validator
+             * (see below). We just need to update the regex.
+             * The preLink fn guaranetees our observer is called first.
+             */
+            if (isString(regex) && regex.length > 0) {
+              regex = new RegExp('^' + regex + '$');
+            }
+
+            if (regex && !regex.test) {
+              //The built-in validator will throw at this point
+              return;
+            }
+
+            regexp = regex || undefined;
+          });
+
+        },
+        post: function(scope, elm, attr, ctrl) {
+          if (!ctrl) return;
+
+          regexp, patternExp = attr.ngPattern || attr.pattern;
+
+          //The postLink fn guarantees we overwrite the built-in pattern validator
+          ctrl.$validators.pattern = function(value) {
+            return ctrl.$isEmpty(value) ||
+              isUndefined(regexp) ||
+              regexp.test(value);
+          };
+        }
+      };
+    }
+  };
+});
+```
+
 
 ### form