minor #18807 [HttpKernel] Auto-register kernel as an extension (alexandre-daubois)

This PR was merged into the 5.4 branch.

Discussion
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[HttpKernel] Auto-register kernel as an extension

Fix #14278

Commits
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e59d643 [HttpKernel] Auto-register kernel as an extension

Author: javiereguiluz

configuration/micro_kernel_trait.rst

@@ -105,7 +105,18 @@ Adding Interfaces to "Micro" Kernel
 When using the ``MicroKernelTrait``, you can also implement the
 ``CompilerPassInterface`` to automatically register the kernel itself as a
 compiler pass as explained in the dedicated
-:ref:`compiler pass section <kernel-as-compiler-pass>`.
+:ref:`compiler pass section <kernel-as-compiler-pass>`. If the
+:class:`Symfony\\Component\\DependencyInjection\\Extension\\ExtensionInterface`
+is implemented when using the ``MicroKernelTrait``, then the kernel will
+be automatically registered as an extension. You can learn more about it in
+the dedicated section about
+:ref:`managing configuration with extensions <components-dependency-injection-extension>`.
+
+.. versionadded:: 5.2
+
+    The automatic registration of the kernel as an extension when implementing the
+    :class:`Symfony\\Component\\DependencyInjection\\Extension\\ExtensionInterface`
+    was introduced in Symfony 5.2.
 
 It is also possible to implement the ``EventSubscriberInterface`` to handle
 events directly from the kernel, again it will be registered automatically::