Added comments to VCL template files describing the purpose / use of the X-Pool header

Author: davidalger

app/code/Magento/PageCache/etc/varnish4.vcl

@@ -26,6 +26,9 @@ sub vcl_recv {
         if (client.ip !~ purge) {
             return (synth(405, "Method not allowed"));
         }
+        # To use the X-Pool header for purging varnish during automated deployments, make sure the X-Pool header
+        # has been added to the response in your backend server config. This is used, for example, by the
+        # capistrano-magento2 gem for purging old content from varnish during it's deploy routine.
         if (!req.http.X-Magento-Tags-Pattern && !req.http.X-Pool) {
             return (synth(400, "X-Magento-Tags-Pattern or X-Pool header required"));
         }

app/code/Magento/PageCache/etc/varnish5.vcl

@@ -27,6 +27,9 @@ sub vcl_recv {
         if (client.ip !~ purge) {
             return (synth(405, "Method not allowed"));
         }
+        # To use the X-Pool header for purging varnish during automated deployments, make sure the X-Pool header
+        # has been added to the response in your backend server config. This is used, for example, by the
+        # capistrano-magento2 gem for purging old content from varnish during it's deploy routine.
         if (!req.http.X-Magento-Tags-Pattern && !req.http.X-Pool) {
             return (synth(400, "X-Magento-Tags-Pattern or X-Pool header required"));
         }