CHEF-2730 explain what flush_cache in yum actually does.

The documentation for flush_cache was misleading, and many assumed it ran
'yum clean all' as apposed to actually discarding chef's yum-dump.py
output. This expels that myth and should stop others thinking the same.

See https://github.com/opscode/chef/commit/5268746fee205eb40e96a2437172580fe8ac3fee for a bit more details.