minor #7459 add warning about auto_alias is not enabled by default (mlebkowski)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7459).

Discussion
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add warning about `auto_alias` is not enabled by default

The `auto_alias` feature is described as if it were enabled out of the box

refs symfony/symfony#21321

Commits
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317b6ed add warning about `auto_alias` is not enabled by default

Author: xabbuh

reference/dic_tags.rst

@@ -353,6 +353,11 @@ wrapping their names with ``%`` characters).
     sense most of the times to prevent accessing those services directly instead
     of using the generic service alias.
 
+.. note::
+
+    You need to manually add the ``Symfony\Component\DependencyInjection\Compiler\AutoAliasServicePass``
+    compiler pass to the container for this feature to work.
+
 console.command
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