minor #5443 Added a note about the implementation of the verbosity semantic methods (javiereguiluz)

This PR was submitted for the 2.7 branch but it was merged into the 2.6 branch instead (closes #5443).

Discussion
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Added a note about the implementation of the verbosity semantic methods

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6+
| Fixed tickets | #5123

Commits
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a88d1c6 Reworded a note about BC
28b98b9 Added a note about the implementation of the verbosity semantic methods

Author: weaverryan

components/console/introduction.rst

@@ -223,6 +223,13 @@ verbosity levels::
         // ...
     }
 
+.. note::
+
+    These semantic methods are defined in the ``OutputInterface`` starting from
+    Symfony 3.0. In previous Symfony versions they are defined in the different
+    implementations of the interface (e.g. :class:`Symfony\\Component\\Console\\Output\\Output`)
+    in order to keep backwards compatibility.
+
 When the quiet level is used, all output is suppressed as the default
 :method:`Symfony\\Component\\Console\\Output\\Output::write` method returns
 without actually printing.