Better documentation on how to allow Socket Connections

Since we are updating the README file, it might good to write down somewhere that in PHP 8.1 and above, we get the following error when using the common setup of $settings['redis.connection']['port'] = NULL;:
Deprecated function: Redis::connect(): Passing null to parameter #2 ($port) of type int is deprecated in Drupal\redis\Client\PhpRedis->getClient() (line 32 of /XXX/modules/contrib/redis/src/Client/PhpRedis.php).

I checked how Matomo handled this and it seems that the suggested way is to use the following instead:

$settings['redis.connection']['port'] = 0;

Just ran into this case myself and the information in this issue was very helpful. So let's get this finished :)

#15 is correct, I'll prepare a MR based on #13 and #15.

Here we go. Please review the MR.

I decided against all the magic from #5 and #11 because I think both might be confusing or even unexpected. So I think we should just explicitly document this.

Please review and let's get this fixed after 8 years! :D

I totally agree that we shouldn't go on with any changes in the code. Just document. And your MR succeeds in this.
So, I set it as RTBC ;)