src: add comment for TicketKeyCallback

Fixes: https://github.com/nodejs/node/issues/13176
PR-URL: https://github.com/nodejs/node/pull/13193


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>