minor #10985 [UI] Flash Message keys (Guikingone)

This PR was merged into the 3.4 branch.

Discussion
----------

[UI] Flash Message keys

Hi everyone,

It seems that the doc lacks the case when we use multiple keys in order to ease the `for` tag.

Commits
-------

9516b15 feat(ui): flash message keys added

Author: javiereguiluz

controller.rst

@@ -546,7 +546,16 @@ read any flash messages from the session using ``app.flashes()``:
         </div>
     {% endfor %}
 
-    {# ...or you can read and display every flash message available #}
+    {# ...or you can read and display every flash message available for given labels #}
+    {% for label, messages in app.flashes(['success', 'warning']) %}
+        {% for message in messages %}
+            <div class="flash-{{ label }}">
+                {{ message }}
+            </div>
+        {% endfor %}
+    {% endfor %}
+
+    {# ...finally you can read and display every flash message available #}
     {% for label, messages in app.flashes %}
         {% for message in messages %}
             <div class="flash-{{ label }}">