doc: explain backport labels

PR-URL: https://github.com/nodejs/node/pull/40520


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>