updating blt docs to cover ACSF memcache use. (#3136)

docs/memcache.md

@@ -10,6 +10,19 @@ Before enabling the Memcache module, it is important to understand how the Drupa
 
 BLT modifies the Memcache module integration on Acquia Cloud. BLT's configuration explicitly overrides the default bins for the discovery, bootstrap, and config cache bins because Drupal core permanently caches these static bins by default. This is required for rebuilding service definitions accurately on cache rebuilds and deploys. See [caching.settings.php](/settings/cache.settings.php).
 
+## Acquia Cloud Site Factory
+
+As of BLT 9.2, the factory hooks contain the necessary code to handle memcache integration with ACSF provided that your subscription and hardware are properly configured. [Using Memcached on Acquia Cloud](https://docs.acquia.com/acquia-cloud/performance/memcached/) provides additional information about this.
+
+If you are upgrading from a previous version of BLT to 9.2.x, make sure and re-generate your factory hooks using:
+
+```
+recipes:acsf:init:hooks
+``` 
+
+This will create a new memcache factory hook for use on ACSF.
+
+
 ## Local Development
 
 The below has been tested with DrupalVM as configured through BLT's `blt vm` command, but should also work for most CI environments where the memcache backend is localhost on port 11211.