minor symfony#27657 [ExpressionLanguage] change evaluate() docblock return type from string to mixed (jspee)

This PR was submitted for the 4.1 branch but it was merged into the 2.8 branch instead (closes symfony#27657).

Discussion
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[ExpressionLanguage] change `evaluate()` docblock return type from string to mixed

Issue: symfony#27652

Commits
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2d26a55 change `evaluate()` docblock return type from string to mixed

Author: fabpot

src/Symfony/Component/ExpressionLanguage/ExpressionLanguage.php

@@ -60,7 +60,7 @@ public function compile($expression, $names = array())
      * @param Expression|string $expression The expression to compile
      * @param array             $values     An array of values
      *
-     * @return string The result of the evaluation of the expression
+     * @return mixed The result of the evaluation of the expression
      */
     public function evaluate($expression, $values = array())
     {