DOC: More clarification in loadData.

ukbparse/importing.py

@@ -500,8 +500,9 @@ def loadData(datafiles,
     :arg vartable:      Variable table
 
     :arg columns:       Dict of ``{ file : [Column] }`` mappings,
-                        defining the columns to load (as returned by
-                        :func:`columnsToLoad`).
+                        defining the columns to load, as returned by
+                        :func:`columnsToLoad`. It is assumed that the
+                        first column in each list is the index column.
 
     :arg nrows:         Number of rows to read at a time. Defaults to
                        :attr:`NUM_ROWS`.