Clarify signed requests in the ESI renderer

Author: wouterj

book/http_cache.rst

@@ -1207,6 +1207,11 @@ One great advantage of the ESI renderer is that you can make your application
 as dynamic as needed and at the same time, hit the application as little as
 possible.
 
+.. caution::
+
+    The fragment listener only responds to signed requests. Requests are only
+    signed when using fragment renderer and the ``render_esi`` Twig function.
+
 .. note::
 
     Once you start using ESI, remember to always use the ``s-maxage``