Add a note in the docs about disabling wpt-importer

Bug: 755756
Change-Id: Ia53261df2ef3fb8030b0349b6498aa32d02e9347
Reviewed-on: https://chromium-review.googlesource.com/617593
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495564}

docs/testing/web_platform_tests.md

@@ -44,7 +44,11 @@ The easiest way to check the status of recent imports is to look at:
 -   Recent CLs created by
     [blink-w3c-test-autoroller@chromium.org](https://chromium-review.googlesource.com/q/owner:blink-w3c-test-autoroller%40chromium.org).
 
-Automatic imports are intended to run at least once every 24 hours.
+The import jobs will generally be green if either there was nothing to do,
+or a CL was successfully submitted.
+
+If the importer starts misbehaving, it could be disabled by turning off the
+auto-import mode by landing [this CL](https://crrev.com/c/617479/).
 
 ### Failures caused by automatic imports.