docs(input[number]): mention incompatibility with allowInvalid

Closes angular#11390

Author: Narretz

Diff for: src/ng/directive/input.js

@@ -612,6 +612,16 @@ var inputType = {
    * error docs for more information and an example of how to convert your model if necessary.
    * </div>
    *
+   * ## Issues with HTML5 constraint validation
+   *
+   * In browsers that follow the
+   * [HTML5 specification](https://html.spec.whatwg.org/multipage/forms.html#number-state-%28type=number%29),
+   * `input[number]` does not work as expected with {@link ngModelOptions `ngModelOptions.allowInvalid`}.
+   * If a non-number is entered in the input, the browser will report the value as an empty string,
+   * which means the view / model values in `ngModel` and subsequently the scope value
+   * will also be an empty string.
+   *
+   *
    * @param {string} ngModel Assignable angular expression to data-bind to.
    * @param {string=} name Property name of the form under which the control is published.
    * @param {string=} min Sets the `min` validation error key if the value entered is less than `min`.