added note about inventory_dir new behaviour (#30865)

* added note about inventory_dir new behaviour

docs/docsite/rst/porting_guide_2.4.rst

@@ -20,6 +20,17 @@ Python version
 
 Ansible will not support Python 2.4 nor 2.5 on the target hosts anymore. Going forward, Python 2.6+ will be required on targets, as already is the case on the controller.
 
+
+Inventory
+=========
+
+Inventory has been refactored to be implemented via plugins and now allows for multiple sources. This change is mostly transparent to users. 
+
+One exception is the ``inventory_dir``, which is now a host variable; previously it could only have one value so it was set globally. This means you cannot use it early in plays anymore to determine ``hosts:`` or similar keywords.
+
+The ``inventory_file`` remains unchaged, as it was always host specific.
+
+
 Deprecated
 ==========