[12.x] PhpRedis Retry & Backoff configuration docs

[mateusjatenee]

I wasn't sure if we should remove the earlier mention to the retry options:

In addition to the default configuration options, PhpRedis supports the following additional connection parameters: name, persistent, persistent_id, prefix, read_timeout, retry_interval, max_retries, backoff_algorithm, backoff_base, backoff_cap, timeout, and context. You may add any of these options to your Redis server configuration in the config/database.php configuration file:

[taylorotwell]

Drafting until release day.