LU-930 doc: improve mkfs.lustre --fsname description

LU-930 doc: improve mkfs.lustre --fsname description

Specify what valid values for the --fsname option are and add some
additional description for this option and --index.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I256c97aaa51cec18cef44b3f34466c4fdb3ebbe5
Reviewed-on: https://review.whamcloud.com/34433
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>
Tested-by: Jenkins
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>