docs(input[radio]): clarify difference between value and ngValue

Closes #7971

Author: Narretz

src/ng/directive/input.js

@@ -913,12 +913,15 @@ var inputType = {
    * HTML radio button.
    *
    * @param {string} ngModel Assignable angular expression to data-bind to.
-   * @param {string} value The value to which the expression should be set when selected.
+   * @param {string} value The value to which the `ngModel` expression should be set when selected.
+   *    Note that `value` only supports `string` values, i.e. the scope model needs to be a string,
+   *    too. Use `ngValue` if you need complex models (`number`, `object`, ...).
    * @param {string=} name Property name of the form under which the control is published.
    * @param {string=} ngChange Angular expression to be executed when input changes due to user
    *    interaction with the input element.
-   * @param {string} ngValue Angular expression which sets the value to which the expression should
-   *    be set when selected.
+   * @param {string} ngValue Angular expression to which `ngModel` will be be set when the radio
+   *    is selected. Should be used instead of the `value` attribute if you need
+   *    a non-string `ngModel` (`boolean`, `array`, ...).
    *
    * @example
       <example name="radio-input-directive" module="radioExample">