clarify sortingStrategy index relation

Author: ben-kalmus

specs/composition/common/schemas/components/Composition.yml

@@ -25,9 +25,9 @@ composition:
 sortingStrategy:
   type: object
   description: |
-    A mapping of sorting labels to the indices (or replicas) that implement those sorting rules.
-
+    A mapping of sorting labels to the indices (or replicas) that implement those sorting rules. The sorting indices MUST be related to the associated main targeted index in the composition.
     Each key is the label your frontend sends at runtime (for example, "Price (asc)"), and each value is the name of the index that should be queried when that label is selected.
+
     When a request includes a "sortBy" parameter, the platform looks up the corresponding index in this mapping and uses it to execute the query. The main injection targeted index is replaced
     with the sorting strategy index it is mapped to.