LU-8541 ldlm: describe lru_max_age better

LU-8541 ldlm: describe lru_max_age better

Add a descripton of the LDLM lru_max_age parameter, including
the (new in 2.11) ability to specify a suffix of "s" to indicate
the units are in seconds.

Improve the description and formatting of lru_size as well.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I70d1193cb5096dd1139ba85d45ab26566209c2ff
Reviewed-on: https://review.whamcloud.com/33406
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>
Tested-by: Jenkins