LU-5502 lod: add a high-level description of the LOD layer

LU-5502 lod: add a high-level description of the LOD layer

Add a high-level description of the LOD layer to lustre/lod/lod_dev.c.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I903394e96a2792683351d741614e2d326e77879d
Reviewed-on: http://review.whamcloud.com/11551
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>