docs: from_parent - add minimal before/after example

Author: RonnyPfannschmidt

doc/en/deprecations.rst

@@ -42,6 +42,10 @@ Node Construction changed to ``Node.from_parent``
 The construction of nodes new should use the named constructor ``from_parent``.
 This limitation in api surface intends to enable better/simpler refactoring of the collection tree.
 
+This means that instead of :code:``MyItem(name="foo", parent=collector, obj=42)``
+one now has to invoke :code:``MyItem.from_parent(collector, name="foo")``.
+
+
 
 ``junit_family`` default value change to "xunit2"
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