Add Opcache note in cache section

[bigb06]

It can be useful to explain that if file cache is enabled, it can waste quickly opcache memory.
Cache folder may be exclude via opcache.blacklist_filename

https://smarty-php.github.io/smarty/5.x/api/caching/basics/

[wisskid]

Is this still a thing after #1007 was fixed in v5.4.0?