minor #10062 [PHPUnitBridge] Explain how to show stack traces (dunglas)

This PR was submitted for the 3.4 branch but it was squashed and merged into the 2.8 branch instead (closes #10062).

Discussion
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[PHPUnitBridge] Explain how to show stack traces

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Commits
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a6e1282 [PHPUnitBridge] Explain how to show stack traces

Author: xabbuh

components/phpunit_bridge.rst

@@ -181,6 +181,22 @@ the value ``"weak"`` which will make the bridge ignore any deprecation notices.
 This is useful to projects that must use deprecated interfaces for backward
 compatibility reasons.
 
+Display the Full Stack Trace
+----------------------------
+
+By default, the PHPUnit Bridge displays only deprecation messages.
+To show the full stack trace related to a deprecation, set the value of ``SYMFONY_DEPRECATIONS_HELPER``
+to a regular expression matching the deprecation message.
+
+For example, if the following deprecation notice is thrown::
+
+    1x: Doctrine\Common\ClassLoader is deprecated.
+      1x in EntityTypeTest::setUp from Symfony\Bridge\Doctrine\Tests\Form\Type
+
+Running the following command will display the full stack trace::
+
+    SYMFONY_DEPRECATIONS_HELPER='/Doctrine\\Common\\ClassLoader is deprecated\./' ./vendor/bin/simple-phpunit
+
 Time-sensitive Tests
 ---------------------