LU-930 doc: update lustre.7 man page

LU-930 doc: update lustre.7 man page

Update the lustre.7 man page to better describe current functionality.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I979841e597fcfa8448c708dd66d4d89d3018b1cc
Reviewed-on: https://review.whamcloud.com/45493
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Rick Mohr <mohrrf@ornl.gov>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>