docs(ngReadonly): note input type restrictions

Closes #14816

Author: Narretz

src/ng/directive/attrs.js

@@ -230,9 +230,11 @@
  *
  * @description
  *
- * Sets the `readOnly` attribute on the element, if the expression inside `ngReadonly` is truthy.
+ * Sets the `readonly` attribute on the element, if the expression inside `ngReadonly` is truthy.
+ * Note that `readonly` applies only to `input` elements with specific types. [See the input docs on
+ * MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-readonly) for more information.
  *
- * A special directive is necessary because we cannot use interpolation inside the `readOnly`
+ * A special directive is necessary because we cannot use interpolation inside the `readonly`
  * attribute. See the {@link guide/interpolation interpolation guide} for more info.
  *
  * @example