[documentreview] Document how to get the attention of an horizontal group in order to close horizontal issues

We need guidance to groups on how to properly communicate with horizontal groups when attempting to close horizontal issues. As usual, a Group is allowed to time out and attempts to move forward anyway, but a Group should not rely on the Director to get the attention of an horizontal Group in order to close the loop on horizontal issues.

The use case is:

The Group responded to an horizontal comment but never heard back from the horizontal group. The Group either believe they addressed the issue but didn't get confirmation that it was resolved, or they responded to the comment but are still unsure on how to move forward.

Attempting to capture my understanding on how to communicate:

For PING, communicate with public-privacy@w3.org (from @samuelweiler ).
For i18n, communicate with member-i18n-core@w3.org (from @r12a).
For APA, is it public-apa@w3.org (question for @michael-n-cooper )
For Security, what is it?
For TAG, what is it? (question for @ylafon )