LU-5502 osp: add a high-level description of the OSP module

LU-5502 osp: add a high-level description of the OSP module

Add a high-level description of the OSP module to
lustre/osp/osp_dev.c.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I4dfcc3629032377aa7fa726b43fedb24680e5829
Reviewed-on: http://review.whamcloud.com/11562
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>