test: add documentation for common.enoughTestCpu

PR-URL: https://github.com/nodejs/node/pull/31931


Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>