MDL-37145 caching: noted recommendation for dedicated memcache servers
Several reports have been made about the unexpected purging of entire
memcached servers when MUC is configured to use Memcache and caches
get purged.
This is expected and occurs because the memcache extenions don't
provide isolated containers. Moodle data is stored along side any other
data in the cache and when caches get purged all data gets removed
whether it belongs to Moodle or not.
The docs cover this point and here I've added notes about this to both
config-dist.php and cache/README.md
