doc: add text about moving long commit lists out of PR description

PR-URL: https://github.com/nodejs/node/pull/39186


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>