minor #16078 [Filesystem] Add third argument $lock to Filesystem::appendToFile (zairigimad)

This PR was submitted for the 5.3 branch but it was merged into the 5.4 branch instead.

Discussion
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[Filesystem] Add third argument $lock to `Filesystem::appendToFile`

related issue #16028

Commits
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78e6f65 Update filesystem.rst

Author: javiereguiluz

components/filesystem.rst

@@ -311,10 +311,16 @@ The ``file.txt`` file contains ``Hello World`` now.
 contents at the end of some file::
 
     $filesystem->appendToFile('logs.txt', 'Email sent to user@example.com');
+    // with the third argument set to true you can lock the file when writing to it.
+    $filesystem->appendToFile('logs.txt', 'Email sent to user@example.com', true);
 
 If either the file or its containing directory doesn't exist, this method
 creates them before appending the contents.
 
+.. versionadded:: 5.4
+
+    The third argument ``$lock`` was introduced in Symfony 5.4.
+
 Error Handling
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