staging: lustre: ldlm: update comments about ldlm l_flags

Add and update documentation about some of the ldlm l_flags.

Signed-off-by: Bruce Korb <bruce.korb@gmail.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2906


Reviewed-by: Keith Mannthey <Keith.Mannthey@intel.com>
Reviewed-on: http://review.whamcloud.com/7963


Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>