LUDOC-391 audit: doc for Lustre Audit with Changelogs

LUDOC-391 audit: doc for Lustre Audit with Changelogs

This patch adds the documentation for the Lustre Audit with Changelogs
feature created in LU-9727.
This doc is added under the Lustre Monitoring section, along with
the Changelogs documentation.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ief72f277f148d226b462f18a06efe59e832fc065
Reviewed-on: https://review.whamcloud.com/30821
Tested-by: Jenkins
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>