Documentation: Update stale SMP cache_dir caveats (#1394)

Documentation: Update stale SMP cache_dir caveats (#1394)

The requirement to specify "workers" before "cache_dir" was added in
2010 commit acf69d7. It became obsolete since 2011 commit 095ec2b.

The "dedicated cache directory" hack for UFS-based stores has always led
to HTTP violations, but the increased complexity of worker-to-worker
synchronization code (required to improve HTTP support) also increased
the probability of crashes or worse outcomes when SMP conditionals are
used. Those hacks violate the "all processes see the same configuration"
and similar basic code assumptions. We do not test (and usually do not
even consider the needs of) such unsupported configurations.