[doc] example code for reading the default MD_DIMS

The default value is explained in _metadata.py in English.
Explicitly give the equivalent Python version, as it's not obvious to
come up with it.

Author: pieleric

doc/develop/backend-api.rst

@@ -619,6 +619,14 @@ in a different order. It is possible to override the default dimension order by
 using the ``MD_DIMS`` metadata. For instance, RGB data is often stored with the
 channel as last dimension for display. Such case can be indicated with "YXC".
 
+.. note::
+
+       As a DataArray number of dimension can vary, and by default the MD_DIMS is undefined, to
+       indicate the "default order", it is not directly easy to read this metadata.
+       It is recommended to access it this way:
+
+       ``dims = data.metadata.get(model.MD_DIMS, "CTZYX"[-data.ndim::])``
+
 Convention about measurement units
 ==================================