src: fix comments re PER_ISOLATE macros

PR-URL: https://github.com/nodejs/node/pull/12899


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James Snell <jasnell@gmail.com>