app_python3: update docs for threads_mode parameter

src/modules/app_python3/doc/app_python3_admin.xml

@@ -168,6 +168,10 @@ modparam("app_python3", "child_init_method", "my_child_init")
 			v6.0.x). If set to 0, use PyGILState_Ensure()/PyGILState_Release()
 			(the mode implemented initially).
 	    </para>
+	    <para>
+		If set to 1, Python threading Thread objects created in the KEMI script will be scheduled. Otherwise,
+                additional Python threads will not be scheduled (no release of GIL).
+	    </para>
 	    <para>
 		<emphasis>
 			Default value is <quote>0</quote>.