Added note about on-ignore=null

Author: dantleech

book/service_container.rst

@@ -812,6 +812,12 @@ allow for an optional dependency::
             // ...
         }
 
+.. note::
+
+    Sometimes you may not want to ignore the argument but rather explicitly
+    set it to null. In thiscase you can use the`set (in XML)
+    ``on-ignore=null``.
+
 Core Symfony and Third-Party Bundle Services
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