minor #14806 [PHPUnitBridge] Document the deprecation baseline file (javiereguiluz)

This PR was merged into the 5.2 branch.

Discussion
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[PHPUnitBridge] Document the deprecation baseline file

Fixes #14367.

I have a question about this: the `baselineFile` must be an absolute path in all cases? Can it be relative to project root?

Maybe @alexpott can help us with that question. Thanks!

Commits
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83af9a1 [PhpUnitBridge] Document the deprecation baseline file

Author: javiereguiluz

components/phpunit_bridge.rst

@@ -289,6 +289,36 @@ Here is a summary that should help you pick the right configuration:
 |                        | cannot afford to use one of the modes above.        |
 +------------------------+-----------------------------------------------------+
 
+Baseline Deprecations
+.....................
+
+If your application has some deprecations that you can't fix for some reasons,
+you can tell Symfony to ignore them. The trick is to create a file with the
+allowed deprecations and define it as the "deprecation baseline". Deprecations
+inside that file are ignore but the rest of deprecations are still reported.
+
+First, generate the file with the allowed deprecations (run the same command
+whenever you want to update the existing file):
+
+.. code-block:: terminal
+
+    $ SYMFONY_DEPRECATIONS_HELPER='generateBaseline=true&baselineFile=tests/allowed.json' ./vendor/bin/simple-phpunit
+
+This command stores all the deprecations reported while running tests in the
+given file and encoded in JSON. The file path defined in ``baselineFile`` can
+be absolute or relative to your project root.
+
+Then, you can run the following command to use that file and ignore those deprecations:
+
+.. code-block:: terminal
+
+    $ SYMFONY_DEPRECATIONS_HELPER='baselineFile=tests/allowed.json' ./vendor/bin/simple-phpunit
+
+.. versionadded:: 5.2
+
+    The ``baselineFile`` and ``generateBaseline`` options were introduced in
+    Symfony 5.2.
+
 Disabling the Verbose Output
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~