LU-930 doc: add audit_mode in lctl-nodemap-modify man page

LU-930 doc: add audit_mode in lctl-nodemap-modify man page

audit_mode is a nodemap property added by patch under LU-9727.
It can be set via 'lctl nodemap_modify' command, so add reference
in corresponding man page.

Test-Parameters: trivial
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ic600756ada257e3e2cfe92a1c30e9b7342f2e4d1
Reviewed-on: https://review.whamcloud.com/34088
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>