bug #5418 Import Psr LogLevel (harikt)

This PR was squashed before being merged into the 2.6 branch (closes #5418).

Discussion
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Import Psr LogLevel

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| License       | MIT

A question regarding the output, by default the o/p was not shown to me, and it introduced a bit of confusion whether it is working or not until I read the verbosity .

Is there a way we can improve the same ?

Commits
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ab87e62 Import Psr LogLevel

Author: wouterj

components/console/logger.rst

@@ -84,6 +84,7 @@ The association between the log level and the verbosity can be configured
 through the second parameter of the :class:`Symfony\\Component\\Console\\ConsoleLogger`
 constructor::
 
+    use Psr\Log\LogLevel;
     // ...
     $verbosityLevelMap = array(
         LogLevel::NOTICE => OutputInterface::VERBOSITY_NORMAL,