Backport DOCSP-27076 to v4.4 (#2845)

* (DOCSP-27076): Clarify version behavior for comparison operators in a $lookup (#2821)

* (DOCSP-27076): Remove eq from list of operators that can use a lookup index in 5.0

* remove version reference

* add limitation for range query operators

Author: jeff-allen-mongo

source/includes/expr-operators-and-indexes.rst

@@ -0,0 +1,14 @@
+:expression:`$eq` operators placed in an :query:`$expr` operator can use
+an index on the ``from`` collection referenced in a :pipeline:`$lookup`
+stage. Limitations:
+
+- Range comparison operators (:expression:`$lt`, :expression:`$lte`,
+  :expression:`$gt`, :expression:`$gte`) cannot use the index. 
+
+- :ref:`Multikey indexes <index-type-multikey>` are not used.
+  
+- Indexes are not used for comparisons where the operand is an array or
+  the operand type is undefined.
+  
+- Indexes are not used for comparisons with more than one :ref:`field
+  path <agg-quick-ref-field-paths>` operand.

source/reference/operator/query/expr.txt

@@ -33,18 +33,14 @@ Definition
 Behavior
 --------
 
-- :query:`$expr` can build query expressions that compare fields 
-  from the same document in a :pipeline:`$match` stage.
+:query:`$expr` can build query expressions that compare fields from the
+same document in a :pipeline:`$match` stage.
 
-  - If the :pipeline:`$match` stage is part of a :pipeline:`$lookup`
-    stage, :query:`$expr` can compare fields using ``let`` variables.
-    See :ref:`lookup-multiple-joins` for an example. 
+If the :pipeline:`$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. 
 
-  - :query:`$expr` only uses indexes on the ``from`` collection for
-    equality matches in a :pipeline:`$match` stage.
-
-- :query:`$expr` does not support
-  :ref:`multikey indexes <index-type-multikey>`.
+.. include:: /includes/expr-operators-and-indexes.rst
 
 Examples
 --------