doc: document starpu_data_interface_get_next_id

doc/chapters/advanced-api.texi

@@ -142,6 +142,10 @@ future CRC computation. This is used for computing data size footprint.
 @node An example of data interface
 @subsection An example of data interface
 
+@deftypefun int starpu_data_interface_get_next_id ()
+Returns the next available id for a newly created data interface.
+@end deftypefun
+
 TODO
 See @code{src/datawizard/interfaces/vector_interface.c} for now.