docs(ngPattern): add option of specifying literal RegExp

Closes #15929

src/ng/directive/validators.js

@@ -96,11 +96,12 @@ var requiredDirective = function() {
  * It is most often used for text-based {@link input `input`} controls, but can also be applied to custom text-based controls.
  *
  * The validator sets the `pattern` error key if the {@link ngModel.NgModelController#$viewValue `ngModel.$viewValue`}
- * does not match a RegExp which is obtained by evaluating the AngularJS expression given in the
- * `ngPattern` attribute value:
- * * If the expression evaluates to a RegExp object, then this is used directly.
- * * If the expression evaluates to a string, then it will be converted to a RegExp after wrapping it
- * in `^` and `$` characters. For instance, `"abc"` will be converted to `new RegExp('^abc$')`.
+ * does not match a RegExp which is obtained from the `ngPattern` attribute value:
+ * - the value is an AngularJS expression:
+ *   - If the expression evaluates to a RegExp object, then this is used directly.
+ *   - If the expression evaluates to a string, then it will be converted to a RegExp after wrapping it
+ *     in `^` and `$` characters. For instance, `"abc"` will be converted to `new RegExp('^abc$')`.
+ * - If the value is a RegExp literal, e.g. `ngPattern="/^\d+$/"`, it is used directly.
  *
  * <div class="alert alert-info">
  * **Note:** Avoid using the `g` flag on the RegExp, as it will cause each successive search to