Docs/ABI/damon: document quota goal metric file

Update DAMON ABI document for the quota goal target_metric file.

Link: https://lkml.kernel.org/r/20240219194431.159606-17-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/ABI/testing/sysfs-kernel-mm-damon

@@ -229,6 +229,12 @@ Description:	Writing a number 'N' to this file creates the number of
 		directories for setting automatic tuning of the scheme's
 		aggressiveness named '0' to 'N-1' under the goals/ directory.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/quotas/goals/<G>/target_metric
+Date:		Feb 2024
+Contact:	SeongJae Park <sj@kernel.org>
+Description:	Writing to and reading from this file sets and gets the quota
+		auto-tuning goal metric.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/quotas/goals/<G>/target_value
 Date:		Nov 2023
 Contact:	SeongJae Park <sj@kernel.org>