minor #18076 Update data_transformers.rst (hbgamra)

This PR was submitted for the 6.2 branch but it was merged into the 5.4 branch instead.

Discussion
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Update data_transformers.rst

Adding return type for addModelTransformer functions params

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Commits
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2b5b60a Update data_transformers.rst

Author: OskarStark

form/data_transformers.rst

@@ -78,11 +78,11 @@ class::
 
             $builder->get('tags')
                 ->addModelTransformer(new CallbackTransformer(
-                    function ($tagsAsArray) {
+                    function ($tagsAsArray): string {
                         // transform the array to a string
                         return implode(', ', $tagsAsArray);
                     },
-                    function ($tagsAsString) {
+                    function ($tagsAsString): array {
                         // transform the string back to an array
                         return explode(', ', $tagsAsString);
                     }