LU-5502 ofd: add a high level description of the OFD layer

LU-5502 ofd: add a high level description of the OFD layer

Add a high-level description of the OFD module to
lustre/ofd/ofd_dev.c.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Id46a2d24ff2e2834e80e9d0f378ef2b515268bb3
Reviewed-on: http://review.whamcloud.com/11576
Tested-by: Jenkins
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>