bug #4116 Fixed the code snippets for the expression language functions (stof)

This PR was merged into the 2.4 branch.

Discussion
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Fixed the code snippets for the expression language functions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets | n/a

The compilation callable was broken for the example of the lowercase function (in 2 different ways in the different snippet as the first one was attempted to be fixed in #3771 but my comment asking to remove the return statement inside the compiled expression was misunderstood and the other return statement returning the compiled expression was removed as well.).

Commits
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3eaaf26 Fixed the code snippets for the expression language functions

Author: weaverryan

Diff for: components/expression_language/extending.rst

@@ -35,7 +35,7 @@ This method has 3 arguments:
 
     $language = new ExpressionLanguage();
     $language->register('lowercase', function ($str) {
-        is_string(%1$s) ? strtolower(%1$s) : %1$s;
+        return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
     }, function ($arguments, $str) {
         if (!is_string($str)) {
             return $str;
@@ -69,11 +69,7 @@ Override ``registerFunctions`` to add your own functions::
             parent::registerFunctions(); // do not forget to also register core functions
 
             $this->register('lowercase', function ($str) {
-                if (!is_string($str)) {
-                    return $str;
-                }
-
-                return sprintf('strtolower(%s)', $str);
+                return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
             }, function ($arguments, $str) {
                 if (!is_string($str)) {
                     return $str;