minor #6920 [Config] Note about bundle priority for PrependExtensionInterface (wodor)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6920).

Discussion
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[Config] Note about bundle priority for PrependExtensionInterface

I've stumbled upon problem that my config setting wasn't prepended using PrependExtensionInterface. It was because I didn’t know that first bundle takes priority when it comes to overriding exactly the same config key.

Commits
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34d8d48 Note about bundle priority for PrependExtensionInterface

Author: xabbuh

bundles/prepend_extension.rst

@@ -131,3 +131,8 @@ The above would be the equivalent of writing the following into the
             // ...
             'use_acme_goodbye' => false,
         ));
+
+More than one bundle using PrependExtensionInterface
+----------------------------------------------------
+
+When there is more than one bundle prepending the same extension and defining the same key, the bundle that is registered **first** will take priority and next bundles won't override this specific config setting.