Merge pull request #833 from humanmade/make-param-override-clearer

Make param override clearer

docs/page-caching.md

@@ -76,7 +76,9 @@ Non-cacheable requests (such as logged-in users) can safely use these headers.
 
 ## Customizations
 
-The page cache can be customised via your `composer.json`. The example below shows the default configuration:
+The Batcache page cache can be customised via your `composer.json` configuration.
+
+The default configuration is:
 
 ```json
 {
@@ -115,6 +117,8 @@ Some query string parameters have no effect on the content or output of the page
 
 The page cache will filter these parameters out when generating the cache key as specified in the [Cache Key Calculation](#cache-key-calculation) section above, and will cause the cache key to be shared across more pages, improving your cache hit rate.
 
+**Note:** Overriding this configuration option will completely replace the configuration, so if you want to retain any of the default parameters, ensure you repeat them in your custom configuration.
+
 
 ### Headers