DOCS-16614 lookup performance fix (#6080) (#6098)

* DOCS-16614 lookup performance fix:

* DOCS-16614 lookup performance fix

* DOCS-16614 adding note about source collection

* DOCS-16614 adding note about source collection

* DOCS-16614 adding admonotion

* DOCS-16614 adding admonotion

* DOCS-16614 typo

* DOCS-16614 fixing format

* DOCS-16614 fixing format

* DOCS-16614 copy edits

* DOCS-16614 copy edits

* DOCS-16614 copy edits

* DOCS-16614 tech edit

* DOCS-16614 tech edit

Author: ltran-mdb2

source/reference/operator/aggregation/lookup.txt

@@ -541,11 +541,14 @@ 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 ``localField``.
+         single join perform better when the foreign collection contains
+         an index on the ``foreignField``.
 
-       - An index on the ``foreignField`` in the foreign collection
-         cannot support an equality match with a single join.
+         .. 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>`
 
@@ -571,10 +574,7 @@ different ``$lookup`` operations.
      - .. _correlated-subqueries-performance: 
 
        - ``$lookup`` operations that contain correlated subqueries
-         typically perform better when the following conditions apply:
-
-         - The source collection contains an index on the
-           ``localField``.
+         perform better when the following conditions apply:
 
          - The foreign collection contains an index on the
            ``foreignField``.