Document disallowed words in design matrix

The rejected values were undocumented. A cell holding a word such as
NONE is visibly non-empty, so an error calling it empty or invalid is
hard to act on without a list of what is accepted.

Author: ajaust

docs/ert/getting_started/howto/design_matrix.rst

@@ -39,7 +39,8 @@ NOTE: The DESIGN_MATRIX validation is more strict than that of DESIGN2PARAMS.
 Some problems that were previously hidden or resulting in failures during runtime, will now result
 in errors during validation of the configuration:
 
-- Missing values (empty cells) in the design matrix
+- Missing or invalid values in the design matrix
+  (see :ref:`accepted cell values <design_matrix_cell_values>`)
 - Duplicate parameter names in the design matrix
 - Design sheet is empty
 - REAL column must only contain unique, positive integers

docs/ert/reference/configuration/keywords.rst

@@ -360,6 +360,13 @@ the final set of parameters (for example in parameters.txt in real==0) would be:
     as inactive in the ensemble and not run. For example, if the DESIGN_MATRIX only contains realization
     id 3, then the ensemble_size will be four. Here the realizations 0, 1, and 2 will be marked as inactive and not run.
 
+.. _design_matrix_cell_values:
+.. note::
+    Every cell in a used row or column of the design sheet and the default
+    sheet must hold a value. A cell is invalid if it is empty, contains only
+    whitespace, holds a numeric ``NaN``, or holds the text ``NONE``, ``NULL``
+    or ``NAN`` in any casing. Surrounding whitespaces are ignored.
+
 .. _eclbase:
 
 ECLBASE