DDC-2834: [GH-868] Added documentation section for Memcached

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of mnapoli:

Url: #868

Message:

The documentation only talked about Memcache, an old PHP extension that can now be safely replaced by Memcached.

I've add a section of documentation about Memcached because the code to have it working is not the same.

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-868] was closed:
#868

[doctrinebot]

Issue was closed with resolution "Fixed"