merged branch greg0ire/fix_upgrade_prototype_name_option (PR #5833)

This PR was merged into the 2.1 branch.

Commits
-------

6fb4a1b Remove § about prototype_name customization in 2.0
8a347fd fix option name

Discussion
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Fix upgrade prototype name option

UPGRADE-2.1.md

@@ -423,22 +423,13 @@
   * In the collection type's template, the default name of the prototype field
     has changed from `$$name$$` to `__name__`.
 
-    For custom names, dollar signs are no longer prepended and appended. You are
-    advised to prepend and append two underscores wherever you specify a value
-    for the field's "prototype_name" option.
-
-    Before:
-
-    ```
-    $builder->add('tags', 'collection', array('prototype' => 'proto'));
-
-    // results in the name "$$proto$$" in the template
-    ```
-
-    After:
+    You can now customize the name of the prototype field by changin the
+    "prototype_name" option. You are advised to prepend and append two
+    underscores wherever you specify a value for the field's "prototype_name"
+    option.
 
     ```
-    $builder->add('tags', 'collection', array('prototype' => '__proto__'));
+    $builder->add('tags', 'collection', array('prototype_name' => '__proto__'));
 
     // results in the name "__proto__" in the template
     ```