Add proxy client config prerequisite

Author: ddeboer

Resources/doc/features/invalidation.rst

@@ -1,7 +1,8 @@
 Invalidation
 ============
 
-**Prerequisites**: :ref:`configure caching proxy <foshttpcache:proxy-configuration>`.
+**Prerequisites**: :ref:`configure caching proxy <foshttpcache:proxy-configuration>`
+and :doc:`enable proxy client </reference/configuration/proxy-client>`.
 
 By *invalidating* a piece of content, you tell your HTTP caching proxy (Varnish
 or Nginx) to no longer serve it to clients. When next requested, the proxy will

Resources/doc/features/tagging.rst

@@ -2,7 +2,7 @@ Tagging
 =======
 
 **Prerequisites**: :ref:`configure caching proxy for tagging <foshttpcache:varnish_tagging>`
-(Varnish only).
+(Varnish only) and :doc:`enable proxy client </reference/configuration/proxy-client>`.
 
 If your application has many intricate relationships between cached items,
 which makes it complex to invalidate them by route, cache tagging will be