tls: comment about old-style errors

Old style errors are being migrated to internal/errors.js, however, due
to depreciation of _tls_legacy.js, it isn't worth the effort to migrate
and potentially force users to update their code for this error change.

This comment clarifies the reason why this error is not migrated.

PR-URL: https://github.com/nodejs/node/pull/17759
Refs: https://github.com/nodejs/node/issues/17709


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>