[test] Update comment about adding more swarming attributes.

NOTRY=true
TBR=tmrts@chromium.org

Bug: chromium:978771
Change-Id: I997d212344af2e3940d5220349485685ba04040c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1678360Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62406}

infra/testing/PRESUBMIT.py

@@ -33,7 +33,9 @@ SUPPORTED_SWARMING_DIMENSIONS = [
 ]
 
 # This is not an exhaustive list. It only reflects what we currently use. If
-# there's need to specify a different property, just add it here.
+# there's need to specify a different property, add it here and update the
+# properties passed to swarming in:
+# //build/scripts/slave/recipe_modules/v8/testing.py.
 SUPPORTED_SWARMING_TASK_ATTRS = [
   'expiration',
   'hard_timeout',