Describe configuration merging in the upgrade guide

[tontonsb]

The issues that arise because of the automatic configuration merging are very rare, but easy to miss if you run into them. And the fix is hard to discover. Therefore it feels appropriate to add some documentation on that.

I tried to document the disabling option without encouraging it's use.

It appears to mostly affect people doing foreach (config('database.connections') as $connection) or foreach (config('queue.connections') as $connection) which should probably be rewritten to something like

foreach (['core', 'mail'] as $conn) {  // or foreach (config('queue.enabled_connections') as $conn)
    $connection = config('queue.connections')[$conn];
}

but I doubt the issue is common enough to add solution suggestions.

Prev discussions and issues with the change:

• Laravel 11 framework config auto-merging is a risky breaking change framework#52237
• [11.x] Add option to disable merging of base configuration framework#51579
• [L11] Is there a way to disable merging with the base config? framework#50876

[taylorotwell]

I think I'll let it ride without this for now.