[apidoc] qoc indicatorset: Add the notion of an indicator group (2 indicators that need to be shown together)

schemas/IndicatorSet.yaml

@@ -12,7 +12,8 @@ properties:
     format: date
     example: 2023-04-01
   chapters:
-    description: Each chapter has a title and contains 1 or more indicators.
+    description: |-
+        Each chapter has a title and contains 1 or more indicators.
     type: array
     items:
       type: object
@@ -22,7 +23,12 @@ properties:
           type: string
           example: Populatie
         indicators:
-          description: The indicators in this chapter
+          description: |-
+            A chapter contains 1 or more indicator groups. An indicator group is an array of indicators which should
+            always be shown together; they are each other's context. These arrays are not empty, but often contain
+            just one indicator. It is extremely rare for such a set to contain more than 2.
+          type: array
+          items:
             type: array
             items:
               $ref: '#/components/schemas/Indicator'