minor #10115 Add file_relative function to the Twig reference (yceruto, javiereguiluz)

This PR was merged into the master branch.

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Add file_relative function to the Twig reference

Documenting https://github.com/symfony/symfony/pull/27978/files#diff-f1a4cceb03e213bc53ab3f0ed53b7da5R215

Commits
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540817e Final reword
67460dc Reword and added the versionadded directive
3eec2ce Document file_relative Twig function

Author: javiereguiluz

reference/twig_reference.rst

@@ -637,6 +637,30 @@ file_link
 Generates a link to the provided file and line number using
 a preconfigured scheme.
 
+file_relative
+~~~~~~~~~~~~~
+
+.. versionadded::
+    The ``file_relative`` filter was introduced in Symfony 4.2.
+
+.. code-block:: twig
+
+    {{ file|file_relative }}
+
+``file``
+    **type**: ``string``
+
+It transforms the given absolute file path into a new file path relative to
+project's root directory:
+
+.. code-block:: twig
+
+    {{ '/var/www/blog/templates/admin/index.html.twig'|file_relative }}
+    {# if project root dir is '/var/www/blog/', it returns 'templates/admin/index.html.twig' #}
+
+If the given file path is out of the project directory, a ``null`` value
+will be returned.
+
 .. _reference-twig-tags:
 
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