Bug 1896684 - Document test-manifests keyword in taskcluster attributes. r=releng-reviewers,gabriel

Already documented test_manifests attribute is not a valid section name for the test task yml files.

Differential Revision: https://phabricator.services.mozilla.com/D210444

taskcluster/docs/attributes.rst

@@ -526,3 +526,8 @@ Used by android browsertime tasks to track the abi of the product under test.
 apk
 ===
 Used by android browsertime tasks to track the path to the apk of the product under test.
+
+test-manifests
+==============
+
+A list of the test manifests that run in this task.