Reflect the [ugly] reality in external_acl_type cache=n documentation.

Reflect the [ugly] reality in external_acl_type cache=n documentation.

The patch does not change how the cache works, but may help admins
configure the cache correctly if they stumble upon the updated docs.

A typical external ACL cache with the default cache settings consumes
about 70 MB of RAM (more in many cases, e.g., if annotations are used
with external ACLs). Besides memory usage, the default cache is using
only 977 hash buckets for 262144 entries so there may be quite a bit
of linear search going on by default.