Add documentation for file_cache_dir option.
Add documentation for file_cache_dir option.

jmchilton/pulsar,ssorgatem/pulsar,jmchilton/lwr,jmchilton/pulsar,galaxyproject/pulsar,galaxyproject/pulsar,jmchilton/lwr,natefoo/pulsar,natefoo/pulsar,ssorgatem/pulsar