docs(FormCheck): add feedbackType (#6745)

* docs: Add feedbackType to FormCheck

Introduced in commit ab9e080 but not yet documented.

* Update src/FormCheck.tsx

Co-authored-by: Kyle Tsang <6854874+kyletsang@users.noreply.github.com>

---------

Co-authored-by: Kyle Tsang <6854874+kyletsang@users.noreply.github.com>

src/FormCheck.tsx

@@ -120,6 +120,12 @@ const propTypes = {
 
   /** A message to display when the input is in a validation state */
   feedback: PropTypes.node,
+
+  /**
+   * Specify whether the feedback is for valid or invalid fields
+   * @type {('valid'|'invalid')}
+   */
+  feedbackType: PropTypes.oneOf(['valid', 'invalid']),
 };
 
 const FormCheck: BsPrefixRefForwardingComponent<'input', FormCheckProps> =