xfs: start documenting common units and tags used in tracepoints

Because there are a lot of tracepoints that express numeric data with
an associated unit and tag, document what they are to help everyone else
keep these thigns straight.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>