Clarify fastly configuration is optional

config/environments/development.js.example

@@ -180,7 +180,7 @@ var config = {
         ttl: 86400,
         layergroupTtl: 86400 // the max-age for cache-control header in layergroup responses
     }
-    // this configuration enables invalidating by surrogate key in fastly
+    // this [OPTIONAL] configuration enables invalidating by surrogate key in fastly
     ,fastly: {
         // whether the invalidation is enabled or not
         enabled: false,

config/environments/production.js.example

@@ -174,7 +174,7 @@ var config = {
         ttl: 86400,
         layergroupTtl: 86400 // the max-age for cache-control header in layergroup responses
     }
-    // this configuration enables invalidating by surrogate key in fastly
+    // this [OPTIONAL] configuration enables invalidating by surrogate key in fastly
     ,fastly: {
         // whether the invalidation is enabled or not
         enabled: false,

config/environments/staging.js.example

@@ -174,7 +174,7 @@ var config = {
         ttl: 86400,
         layergroupTtl: 86400 // the max-age for cache-control header in layergroup responses
     }
-    // this configuration enables invalidating by surrogate key in fastly
+    // this [OPTIONAL] configuration enables invalidating by surrogate key in fastly
     ,fastly: {
         // whether the invalidation is enabled or not
         enabled: false,

config/environments/test.js.example

@@ -176,7 +176,7 @@ var config = {
         ttl: 86400,
         layergroupTtl: 86400 // the max-age for cache-control header in layergroup responses
     }
-    // this configuration enables invalidating by surrogate key in fastly
+    // this [OPTIONAL] configuration enables invalidating by surrogate key in fastly
     ,fastly: {
         // whether the invalidation is enabled or not
         enabled: false,