minor #5012 #4032 improved comments about em option (raulfraile)

This PR was merged into the 2.3 branch.

Discussion
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#4032 improved comments about em option

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4032

Commits
-------

c5b8926 #4032 improved comments about em option

cookbook/form/data_transformers.rst

@@ -122,7 +122,9 @@ by calling ``addModelTransformer`` (or ``addViewTransformer`` - see
         {
             // ...
 
-            // this assumes that the entity manager was passed in as an option
+            // the "em" is an option that you pass when creating your form. Check out
+            // the 3rd argument to createForm in the next code block to see how this
+            // is passed to the form (also see setDefaultOptions).
             $entityManager = $options['em'];
             $transformer = new IssueToNumberTransformer($entityManager);