doc: describe tls.DEFAULT_MIN_VERSION/_MAX_VERSION

Add documentation for tls.DEFAULT_MAX_VERSION and
tls.DEFAULT_MIN_VERSION, which existed in v10.6.0

Fixes: https://github.com/nodejs/node/issues/28758
Refs: https://github.com/nodejs/node/pull/26821

PR-URL: https://github.com/nodejs/node/pull/28827


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>