feature symfony#8675 Mention the handy ignoreExtraKeys() method (jordanlev)

This PR was merged into the 2.8 branch.

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Mention the handy `ignoreExtraKeys()` method

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c175b0d Mention the handy `ignoreExtraKeys()` method

Author: weaverryan

components/config/definition.rst

@@ -210,6 +210,9 @@ Before defining the children of an array node, you can provide options like:
     If called (with ``false``), keys with dashes are *not* normalized to underscores.
     It is recommended to use this with prototype nodes where the user will define
     a key-value map, to avoid an unnecessary transformation.
+``ignoreExtraKeys()``
+    Allows extra config keys to be specified under an array without
+    throwing an exception.
 
 A basic prototyped array configuration can be defined as follows::