minor #5076 Better explain that form types should be unique in the application (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Better explain that form types should be unique in the application

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4960

Commits
-------

7a78532 Fixed a minor grammar issue
0eb149b Recommend app_ instead of app. as form type prefix
52a5551 Recommend a better way to standardize application form types
d47e751 Fixed syntax issues and provided more information
12b77af Better explain that form types should be unique in the application

Author: weaverryan

book/forms.rst

@@ -1057,9 +1057,16 @@ that will house the logic for building the task form::
         }
     }
 
-This new class contains all the directions needed to create the task form
-(note that the ``getName()`` method should return a unique identifier for this
-form "type"). It can be used to quickly build a form object in the controller::
+.. caution::
+
+    The ``getName()`` method returns the identifier of this form "type". These
+    identifiers must be unique in the application. Unless you want to override
+    a built-in type, they should be different from the default Symfony types
+    and from any type defined by a third-party bundle installed in your application.
+    Consider prefixing your types with ``app_`` to avoid identifier collisions.
+
+This new class contains all the directions needed to create the task form. It can
+be used to quickly build a form object in the controller::
 
     // src/AppBundle/Controller/DefaultController.php