improved GridView docs about empty cell

fixes #9159

framework/grid/GridView.php

@@ -185,7 +185,13 @@ class GridView extends BaseListView
      */
     public $columns = [];
     /**
-     * @var string the HTML display when the content of a cell is empty
+     * @var string the HTML display when the content of a cell is empty.
+     * This property is used to render cells that have no defined content,
+     * e.g. empty footer or filter cells.
+     *
+     * Note that this is not used by the [[DataColumn]] if a data item is `null`. In that case
+     * the [[\yii\i18n\Formatter::nullDisplay|nullDisplay]] property of the [[formatter]] will
+     * be used to indicate an empty data value.
      */
     public $emptyCell = ' ';
     /**