DOCS Filesystem capacity issue potential on caches

Context: silverstripe#6678

docs/en/00_Getting_Started/00_Server_Requirements.md

@@ -137,6 +137,17 @@ Depending on your hosting and deployment mechanisms,
 you may need to configure the plugin to copy files instead.
 See [silverstripe/vendor-plugin](https://github.com/silverstripe/vendor-plugin) for details.
 
+### Caches
+
+Silverstripe relies on various [caches](https://docs.silverstripe.org/en/4/developer_guides/performance/caching/)
+to achieve performant responses. By default, those caches are stored in a temporary filesystem folder,
+and are not shared between multiple server instances. Alternative cache backends such as Redis can be
+[configured](https://docs.silverstripe.org/en/4/developer_guides/performance/caching/).
+
+While cache objects can expire, when using filesystem caching the files are not actively pruned.
+For long-lived server instances, this can become a capacity issue over time - see
+[workaround](https://github.com/silverstripe/silverstripe-framework/issues/6678).
+
 ### Error pages
 
 The default installation includes [silverstripe/errorpage](https://addons.silverstripe.org/add-ons/silverstripe/errorpage),

docs/en/02_Developer_Guides/08_Performance/01_Caching.md

@@ -25,6 +25,11 @@ executing the action is limited to the following cases when performed via a web
  * A user is logged in with ADMIN permissions
  * An error occurs during startup
 
+Caution: Not all caches are cleared through `flush=1`.
+While cache objects can expire, when using filesystem caching the files are not actively pruned.
+For long-lived server instances, this can become a capacity issue over time - see
+[workaround](https://github.com/silverstripe/silverstripe-framework/issues/6678).
+
 ## Configuration
 
 We are using the [PSR-16](http://www.php-fig.org/psr/psr-16/) standard ("SimpleCache")