docs(ngChecked): note that it shouldn't be used with ngModel

Closes angular#11106

src/ng/directive/attrs.js

@@ -206,6 +206,13 @@
  * @priority 100
  *
  * @description
+ * Sets the `checked` attribute on the element, if the expression inside `ngChecked` is truthy.
+ *
+ * Note that this directive should not be used together with {@link ngModel `ngModel`},
+ * as this can lead to unexpected behavior.
+ *
+ * ### Why do we need `ngChecked`?
+ *
  * The HTML specification does not require browsers to preserve the values of boolean attributes
  * such as checked. (Their presence means true and their absence means false.)
  * If we put an Angular interpolation expression into such an attribute then the
@@ -230,7 +237,7 @@
  *
  * @element INPUT
  * @param {expression} ngChecked If the {@link guide/expression expression} is truthy,
- *     then special attribute "checked" will be set on the element
+ *     then the `checked` attribute will be set on the element
  */