DOCS-11997 Document that $expr cannot use a multikey index

Author: Isabella Siu

source/core/index-multikey.txt

@@ -202,6 +202,11 @@ any position in the ``ratings`` array. Then, MongoDB retrieves these
 documents and filters for documents whose ``ratings`` array equals the
 query array ``[ 5, 9 ]``.
 
+$expr
+~~~~~
+
+:query:`$expr` does not support multikey indexes.
+
 Examples
 --------
 

source/reference/operator/query/expr.txt

@@ -38,7 +38,9 @@ from the same document in a :pipeline:`$match` stage.
 
 If the :pipeline:`$match` stage is part of a :pipeline:`$lookup` stage, 
 :query:`$expr` can compare fields using ``let`` variables. See
-:ref:`lookup-multiple-joins` for an example. 
+:ref:`lookup-multiple-joins` for an example.
+
+:query:`$expr` does not support :ref:`multikey indexes <index-type-multikey>`.
 
 Examples
 --------