minor #6656 Clarify usage of handler channel configuration (shkkmo)

This PR was submitted for the 2.3 branch but it was merged into the 2.7 branch instead (closes #6656).

Discussion
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Clarify usage of handler channel configuration

See: https://github.com/symfony/monolog-bundle/blob/master/DependencyInjection/MonologExtension.php#L88

I believe this applies to all versions of Symfony, so should be merged into the branches of documentation for all of them.

Commits
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9b4d123 Clarify usage of handler channel configuration

Author: wouterj

cookbook/logging/channels_handlers.rst

@@ -95,6 +95,10 @@ in all environments, or just ``config_prod.yml`` to happen only in ``prod``:
             ),
         ));
 
+.. caution::
+
+    The 'channels' configuration only works for top level handlers. Handlers that are nested inside a Group, Buffer, Filter, FingersCrossed, or other such Handler will ignore this configuration and will process every message passed to them.
+
 YAML Specification
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