Warn about usage of memcached, instead of suggesting it

[lmeyer1]

Here stands https://devdocs.prestashop.com/1.7/scale/optimizations/#7-prestashop-settings :

If you still want to enable it, if you have only 1 front, use APC, and if you have several front, use a unique central memcached server host (not localhost!)

I think, it is clear, that caching with Memcached in Prestashop does not work for now. Therefore, we should not suggest using it (even though it stands If you still want to enable it.

It should read something like:

Warning: do not enable cache using memcached. The current implementation not only is inefficient, but can give rise to unexpected behavior. Never use it on a production environment.