clarify what "equal shape" means

base_classes/NXdata.nxdl.xml

@@ -102,7 +102,8 @@
  When :ref:`AXISNAME_indices </NXdata@AXISNAME_indices-attribute>` is missing for a given :ref:`AXISNAME </NXdata/AXISNAME-field>` field,
  the position(s) of the :ref:`AXISNAME </NXdata/AXISNAME-field>` field name in the :ref:`axes </NXdata@axes-attribute>` attribute
  is(are) used. The shape of an :ref:`AXISNAME </NXdata/AXISNAME-field>` field must be equal to the shape of the
- :ref:`data </NXdata/DATA-field>` dimensions it spans.
+ :ref:`data </NXdata/DATA-field>` dimensions it spans. This means that `AXISNAME.shape[i] == DATA.shape[AXISNAME_indices[i]]`
+ for each `i` in `[0, AXISNAME.ndim)`.
 
  Note that an :ref:`AXISNAME </NXdata/AXISNAME-field>` field can have more than one dimension and can therefore span
  more than one :ref:`data </NXdata/DATA-field>` dimension. Conversely, one :ref:`data </NXdata/DATA-field>` dimension