docs(input[number]): clarify that model must be of type number

The docs also now link through to the error doc, which contains a runnable
example of how to work around this restriction.

Closes angular#11157
Closes angular#11334

Author: petebacondarwin

src/ng/directive/input.js

@@ -587,7 +587,11 @@ var inputType = {
    * Text input with number validation and transformation. Sets the `number` validation
    * error if not a valid number.
    *
-   * The model must always be a number, otherwise Angular will throw an error.
+   * <div class="alert alert-warning">
+   * The model must always be of type `number` otherwise Angular will throw an error.
+   * Be aware that a string containing a number is not enough. See the {@link ngModel:numfmt}
+   * error docs for more information and an example of how to convert your model if necessary.
+   * </div>
    *
    * @param {string} ngModel Assignable angular expression to data-bind to.
    * @param {string=} name Property name of the form under which the control is published.