Add http-only warning in documentation (#52631)

* Add http-only warning in documentation

* Add empty line

---------

Co-authored-by: Alkis Mavridis <[email protected]>

Author: alkismavridis

airflow-core/docs/howto/run-behind-proxy.rst

@@ -63,6 +63,9 @@ To do so, you need to set the following setting in your ``airflow.cfg``::
   `Uvicorn's docs <https://www.uvicorn.org/deployment/#proxies-and-forwarded-headers>`_. For the full options you can pass here.
   (Please note the ``--forwarded-allow-ips`` CLI option does not exist in Airflow.)
 
+- Please make sure your proxy does not enforce http-only status on the Set-Cookie headers.
+  Airflow frontend needs to access the cookies through javascript, and a http-only flag would disturb this functionality.
+
 .. spelling::
 
   Uvicorn