docs: sync Haystack API reference on Docusaurus (#12161)

Co-authored-by: anakin87 <44616784+anakin87@users.noreply.github.com>

Author: HaystackBot

docs-website/reference/haystack-api/evaluators_api.md

@@ -545,7 +545,9 @@ calculate_idcg(gt_docs: list[Document]) -> float
 Calculate the ideal discounted cumulative gain (IDCG) of the ground truth documents.
 
 Ground truth documents whose comparison value cannot be determined (e.g. missing meta key)
-are excluded, since they can never be matched in `calculate_dcg` either. Including them here
+are excluded, since they can never be matched in `calculate_dcg` either. Documents that share
+a comparison value are collapsed to a single relevant item, mirroring `calculate_dcg`, which
+credits each relevant value at most once. Including duplicates or unmatchable documents here
 would inflate the IDCG and make it impossible for NDCG to reach 1.0 for a perfect retrieval.
 
 **Parameters:**