doc: document that NODE_EXTRA_CA_CERTS is read only once

Fixes: https://github.com/nodejs/node/issues/20432

PR-URL: https://github.com/nodejs/node/pull/37562


Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>