LUDOC-169 large_xattr: Added index entries and notes to enhance large_xattr documenta...

LUDOC-169 large_xattr: Added index entries and notes to enhance large_xattr documentation.

Added index entries to Section 5.3.3 "Setting up Lustre File System" and
Section 16.2 "Upgrading Lustre" to make information about large_attr option
to tune2fs and mkfs.lustre easier to find.

Signed-off-by: Linda Bebernes <linda.bebernes@intel.com>
Change-Id: Id6794ecb97d8368f04b32d7d20c4ce4817380316
Reviewed-on: http://review.whamcloud.com/7731
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>