DOCS-11150: lookup blurb for let variables

Author: kay-kim

source/reference/operator/aggregation/lookup.txt

@@ -184,12 +184,15 @@ The :pipeline:`$lookup` takes a document with the following fields:
        document fields, and then reference the variables in the stages
        in the ``pipeline``.
 
-       .. note::
+       To access the ``let`` variables in the ``pipeline``, use the
+       :query:`$expr` operator.
 
-          You cannot access aggregation variables in the
-          :pipeline:`$match` stage which uses the standard MongoDB
-          query operators.
+       .. note::
 
+          The ``let`` variables are accessible by the stages in the
+          ``pipeline``, including additional :pipeline:`$lookup` stages
+          nested in the ``pipeline``.
+ 
    * - ``as``
 
      - Specifies the name of the new array field to add to the input