doc: clarify when not to run CI on docs

Collaborators won't need to run CI on documentation-only changes.

PR-URL: https://github.com/nodejs/node/pull/33101


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>