docs: Add warning on placeholder tags

Author: fsbraun

docs/source/how-to/use-frontend-as-component.rst

@@ -91,3 +91,10 @@ use the ``{% plugin %}`` template tag with each plugin.
 
     Since no plugins are created in the database, plugins relying on their
     instances being available in the database will potentially not work.
+
+
+.. warning::
+
+    Currently, ``{% placeholder %}`` template tags are not supported inside
+    a ``{% plugin %}`` tag. This is because the ``{% plugin %}`` tag does
+    preprocess the content and placeholders are not recognized by django CMS.