LUDOC-68 dirdata: Added documentation for FID, fid-in-dirent, layout EA, linkEA.

LUDOC-68 dirdata:  Added documentation for FID, fid-in-dirent, layout EA, linkEA.

Provided description of features in Chapter 1 "Understanding Lustre" in
"Lustre Storage and I/O" section. Added information about
enabling fid-in-dirent when upgrading from 1.8 to Chapter 16 "Upgrading
Lustre".

Signed-off-by: Linda Bebernes <linda.bebernes@intel.com>
Change-Id: I62ccb4003bdd885c4177becbfad13062c5c27301
Reviewed-on: http://review.whamcloud.com/7844
Tested-by: Hudson
Reviewed-by: Richard Henwood <richard.henwood@intel.com>