btrfs/064: add a comment to the test case header

It appears that the goal of this case was to test balance and
replace concurrency. But these two operations aren't meant to run
concurrently and the replace failing errors are captured in the
seqres.full output. Which are expected errors. To avoid further
confusion, this patch adds comments. The reason to keep this
test case is at the Link.

Link: https://patchwork.kernel.org/patch/11806307/
Reported-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/btrfs/064

@@ -4,9 +4,11 @@
 #
 # FSQA Test No. btrfs/064
 #
-# Run btrfs balance and replace operations simultaneously with fsstress
-# running in background.
-#
+# Run btrfs balance and replace operations simultaneously with fsstress running
+# in the background, check with the scrub if all the blocks are ok.
+# Balance and replace operations are mutually exclusive operations they can't
+# run simultaneously. One of them is expected to fail when the other is running.
+
 seq=`basename $0`
 seqres=$RESULT_DIR/$seq
 echo "QA output created by $seq"
@@ -62,6 +64,8 @@ run_test()
 	$FSSTRESS_PROG $args >/dev/null 2>&1 &
 	fsstress_pid=$!
 
+	# Start both balance and replace in the background.
+	# Either balance or replace shall run, the other fails.
 	echo -n "Start balance worker: " >>$seqres.full
 	_btrfs_stress_balance $SCRATCH_MNT >/dev/null 2>&1 &
 	balance_pid=$!