attestation_data api clarification for Electra (#545)

Co-authored-by: Nico Flaig <[email protected]>

Author: tbenr

apis/validator/attestation_data.yaml

@@ -20,7 +20,9 @@ get:
         $ref: ../../beacon-node-oapi.yaml#/components/schemas/Uint64
     - name: committee_index
       in: query
-      description: "The committee index for which an attestation data should be created."
+      description: |
+        The committee index for which an attestation data should be created. For `slot`s in
+        Electra and later, this parameter MAY always be set to 0.
       required: true
       schema:
         $ref: "../../beacon-node-oapi.yaml#/components/schemas/Uint64"