Add a comment about the csp_nonce() Twig function

Author: nicwortel

frontend/asset_mapper.rst

@@ -1079,6 +1079,7 @@ it in the CSP header, and then pass the same nonce to the Twig function:
 
 .. code-block:: twig
 
+    {# the csp_nonce() function is defined by the NelmioSecurityBundle #}
     {{ importmap('app', {'nonce': csp_nonce('script')}) }}
 
 The AssetMapper Component Caching System in dev