update docsite about new variable inventory_file

docsite/latest/rst/playbooks2.rst

@@ -148,7 +148,7 @@ period, without the rest of the domain.
 
 Don't worry about any of this unless you think you need it.  You'll know when you do.
 
-Also available, *inventory_dir* is the pathname of the directory holding Ansible's inventory host file.
+Also available, *inventory_dir* is the pathname of the directory holding Ansible's inventory host file, *inventory_file* is the pathname and the filename pointing to the Ansible's inventory host file.
 
 Variable File Separation
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