DOCSP-36128 updating lookup performance (#6139) (#6251)

* DOCSP-36128 updating lookup performance

* DOCSP-36128 updating lookup performance

Author: ltran-mdb2

source/reference/operator/aggregation/lookup.txt

@@ -557,13 +557,9 @@ different ``$lookup`` operations.
 
      - .. _uncorrelated-subqueries-performance:
 
-       - An index on the foreign collection cannot support uncorrelated
-         subqueries.
-      
-       - To improve performance for uncorrelated subqueries, use generic
-         indexing strategies and query optimization techniques. For
-         example, add appropriate indexes to support aggregation stages
-         before ``$lookup``.
+       - ``$lookup`` operations that contain uncorrelated subqueries
+         perform better when the inner pipeline can reference an
+         index of the foreign collection.
 
        - MongoDB only needs to run the ``$lookup`` subquery once before
          caching the query because there is no relationship between the