DOCS-16616 lookup performance fix for v4.4 (#6082)

* DOCS-16616 lookup performance fix for v4.4

* DOCS-16616 lookup performance fix for v4.4

* DOCS-16616 aligning changes with DOCS-16614

* DOCS-16616 aligning changes with DOCS-16614

Author: ltran-mdb2

source/reference/operator/aggregation/lookup.txt

@@ -330,8 +330,14 @@ different ``$lookup`` operations.
      - .. _equality-match-performance:
 
        - ``$lookup`` operations that perform equality matches with a
-         single join typically perform better when the source collection
-         contains an index on the ``foreignField``.
+         single join perform better when the foreign collection contains
+         an index on the ``foreignField``.
+
+         .. important::
+
+            If a supporting index on the ``foreignField`` does not
+            exist, a ``$lookup`` operation that performs an equality
+            match with a single join will likely have poor performance.
 
    * - :ref:`Uncorrelated Subqueries <lookup-uncorrelated-subquery>`