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>

lib/_tls_legacy.js

@@ -608,6 +608,8 @@ function onhandshakestart() {
     // state machine and OpenSSL is not re-entrant. We cannot allow the user's
     // callback to destroy the connection right now, it would crash and burn.
     setImmediate(function() {
+      // Old-style error is not being migrated to the newer style
+      // internal/errors.js because _tls_legacy.js has been deprecated.
       var err = new Error('TLS session renegotiation attack detected');
       if (self.cleartext) self.cleartext.emit('error', err);
     });