feature #7517 [ExpressionLanguage] Document ExpressionFunction::fromPhp (maidmaid, javiereguiluz)

This PR was merged into the master branch.

Discussion
----------

[ExpressionLanguage] Document ExpressionFunction::fromPhp

cf symfony/symfony#21122

Commits
-------

415c4ec Update extending.rst
478400a Minor reword
059155f Add versionadded
6c9e09d Fix typo
c70ea7f Fix typo
6d9af37 Add fromPhp tip

Author: xabbuh

components/expression_language/extending.rst

@@ -90,6 +90,21 @@ register.
         }
     }
 
+.. tip::
+
+    To create an expression function from a PHP function with the
+    :method:`Symfony\\Component\\ExpressionLanguage\\ExpressionFunction::fromPhp` static method::
+
+        ExpressionFunction::fromPhp('strtoupper');
+
+    Namespaced functions are supported, but they require a second argument to
+    define the name of the expression::
+
+        ExpressionFunction::fromPhp('My\strtoupper', 'my_strtoupper');
+
+    .. versionadded:: 3.3
+        The ``ExpressionFunction::fromPhp()`` method was introduced in Symfony 3.3.
+
 You can register providers using
 :method:`Symfony\\Component\\ExpressionLanguage\\ExpressionLanguage::registerProvider`
 or by using the second argument of the constructor::