doc: document NODE_TLS_REJECT_UNAUTHORIZED

This commit documents the NODE_TLS_REJECT_UNAUTHORIZED
environment variable so that the world can know how
potentially dangerous it is.

PR-URL: https://github.com/nodejs/node/pull/24289
Fixes: https://github.com/nodejs/node/issues/24284


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>