Added note about not sharing cache_dir between instances.

config/vufind/config.ini

@@ -1116,7 +1116,10 @@ HMACkey = mySuperSecretValue
 ;ttl = 0
 ; This setting can be used to force caching in a specific location other than the
 ; default location inside the local settings directory. This directory must exist
-; and be writable by the web server.
+; and be writable by the web server. Do not share this directory between multiple
+; instances of VuFind or you may encounter unexpected side effects -- while this
+; directory may be outside of the local settings directory, there should be exactly
+; one separate cache location per local settings directory.
 ;cache_dir = "local/cache/"
 ; Override umask for cache directories and files.
 ;umask = 022