Add warning about multiple modals not supported out of box (react-bootstrap#3142)

Addresses react-bootstrap#3130.

Author: timhwang21

www/src/pages/components/modal.js

@@ -89,6 +89,24 @@ export default function ModalSection({ data }) {
       </p>
       <ReactPlayground codeText={ModalCustomSizing} />
 
+      <h3>
+        <Anchor id="modals-multiple">Multiple Modals</Anchor>
+      </h3>
+      <div className="bs-callout bs-callout-warning">
+        <h4>
+          Not supported
+        </h4>
+        <p>
+          React-Bootstrap modals are not designed to support rendering multiple
+          modals simultaneously. You will have to add <code>react-overlays</code> as
+          a dependency and build your own modal component using its{' '}
+          <a href="https://github.com/react-bootstrap/react-overlays/blob/master/src/ModalManager.js">
+            <code>{'<ModalManager/>'}</code>
+          </a>{' '}
+          component, which supports multiple modals.
+        </p>
+      </div>
+
       <h3>
         <Anchor id="modals-props">Props</Anchor>
       </h3>