minor #9775 Document the hidden services and update debug:container docs (javiereguiluz)

This PR was squashed before being merged into the 4.1 branch (closes #9775).

Discussion
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Document the hidden services and update debug:container docs

This fixes #9658.

Commits
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8010808 Document the hidden services and update debug:container docs

Author: javiereguiluz

service_container.rst

@@ -571,6 +571,9 @@ But, you can control this and pass in a different logger:
             # explicitly configure the service
             App\Service\MessageGenerator:
                 arguments:
+                    # the '@' symbol is important: that's what tells the container
+                    # you want to pass the *service* whose id is 'monolog.logger.request',
+                    # and not just the *string* 'monolog.logger.request'
                     $logger: '@monolog.logger.request'
 
     .. code-block:: xml
@@ -612,13 +615,7 @@ For a full list of *all* possible services in the container, run:
 
 .. code-block:: terminal
 
-    php bin/console debug:container --show-private
-
-.. tip::
-
-    The ``@`` symbol is important: that's what tells the container you want to pass
-    the *service* whose id is ``monolog.logger.request``, and not just the *string*
-    ``monolog.logger.request``.
+    $ php bin/console debug:container
 
 .. _services-binding:
 

service_container/debug.rst

@@ -6,17 +6,17 @@ How to Debug the Service Container & List Services
 ==================================================
 
 You can find out what services are registered with the container using the
-console. To show all services and the class for each service, run:
+console. To show all services (public and private) and their PHP classes, run:
 
 .. code-block:: terminal
 
     $ php bin/console debug:container
 
-By default, only public services are shown, but you can also view private services:
+    # add this option to display "hidden services" too (those whose ID starts with a dot)
+    $ php bin/console debug:container --show-hidden
 
-.. code-block:: terminal
-
-    $ php bin/console debug:container --show-private
+.. versionadded:: 4.1
+    Hidden services and the ``--show-hidden`` option were introduced in Symfony 4.1.
 
 To see a list of all of the available types that can be used for autowiring, run: