doc: explicitly doc package.exports is breaking

If package authors don't explicitly include all previously supported
entry points introducing package.exports will be a Semver-Major change.

Add a warning about this behavior and offer two potential solutions
for module authors.

Refs: https://github.com/then/is-promise/issues/20

PR-URL: https://github.com/nodejs/node/pull/33074


Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>