Bug 1509564 [wpt PR 14214] - Document the source email for assignment, a=testonly

Automatic update from web-platform-testsDocument the source email for assignment (#14214)

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wpt-commits: ad865c2f6bd3ce9ab1333eb78df445cd4c805f36
wpt-pr: 14214

testing/web-platform/tests/docs/_reviewing-tests/email.md

@@ -6,6 +6,6 @@ order: 3
 
 See the [GitHub support page](https://help.github.com/articles/about-email-notifications/)
 for how to filter certain types of email notifications. However, the most
-useful `Cc` email address is not listed on that page:
+useful `Cc` email addresses are not listed on that page:
-`review_requested@noreply.github.com` is for when you are added as a reviewer
+- `review_requested@noreply.github.com` when you are added as a (suggested) `reviewer` on a pull request.
-on a pull request.
+- `assign@noreply.github.com` when you are added as the `assignee` (i.e. as _the_ reviewer) on a pull request.