minor #6235 #6232 update forms as services section (backbone87)

This PR was squashed before being merged into the 2.8 branch (closes #6235).

Discussion
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#6232 update forms as services section

this is to reflect recent changes to form type handling since symfony 2.8

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.8+
| Fixed tickets | #6232

Commits
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be54971 #6232 update forms as services section

best_practices/forms.rst

@@ -71,13 +71,9 @@ Registering Forms as Services
 
 You can also
 :ref:`register your form type as a service <form-cookbook-form-field-service>`.
-But this is *not* recommended unless you plan to reuse the new form type in many
-places or embed it in other forms directly or via the
-:doc:`CollectionType </reference/forms/types/collection>`.
-
-For most forms that are used only to edit or create something, registering
-the form as a service is over-kill, and makes it more difficult to figure
-out exactly which form class is being used in a controller.
+This is only needed if your form type requires some dependencies to be injected
+by the container, otherwise it is unnecessary overhead and therefore *not*
+recommended to do this for all form type classes.
 
 Form Button Configuration
 -------------------------