The documentation in config.php.dist says

Communicate with Redis over a secure connection instead of plain TCP.

This setting affects both single host connections as
well as Sentinel mode.

However, that's not what actually happens. We can connect to TLS-enabled sentinels using tls://, but
if they return a TLS-enabled master, then we still try tcp:// rather than tls:// when connecting. That causes an exception to be thrown:

Caused by: Predis\Connection\ConnectionException: Error while reading line from the server. [tcp://172.16.2.104:6379]

This completes the configuration started in #1828 by making it possible to use a wholly TLS-enabled Redis setup with sentinels.