doc,crypto: re-document oaepLabel option

Erroneously removed in https://github.com/nodejs/node/pull/29516.

Fixes: https://github.com/nodejs/node/issues/31810
Refs: https://github.com/nodejs/node/pull/29516

PR-URL: https://github.com/nodejs/node/pull/31825


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>