DOCS-14609 SBE order projection fields

Author: jason-price-mongodb

source/core/document.txt

@@ -199,6 +199,37 @@ Document Size Limit
 Document Field Order
 ~~~~~~~~~~~~~~~~~~~~
 
+Unlike JavaScript objects, the fields in a BSON document are ordered.
+
+Field Order in Queries
+``````````````````````
+
+For queries, the field order behavior is as follows:
+
+- When comparing documents, field ordering is significant. For
+  example, when comparing documents with fields ``a`` and ``b`` in a
+  query:
+  
+  - ``{a: 1, b: 1}`` is equal to ``{a: 1, b: 1}``
+  
+  - ``{a: 1, b: 1}`` is not equal to ``{b: 1, a: 1}``
+
+- For efficient query execution, the query engine may reorder fields
+  during query processing. Among other cases, reordering fields may
+  occur when processing these projection operators:
+  :pipeline:`$project`, :pipeline:`$addFields`, :pipeline:`$set`, and
+  :pipeline:`$unset`.
+  
+  - Field reordering may occur in intermediate results as well as the
+    final results returned by a query.
+  
+  - Because some operations may reorder fields, you should not rely on
+    specific field ordering in the results returned by a query that
+    uses the projection operators listed earlier.
+
+Field Order in Write Operations
+```````````````````````````````
+
 .. include:: /includes/fact-update-field-order.rst
    :start-after: order-of-document-fields
 

source/includes/fact-update-field-order.rst

@@ -1,7 +1,7 @@
 .. order-of-document-fields
 
-MongoDB preserves the order of the document fields following write
-operations *except* for the following cases:
+For write operations, MongoDB preserves the order of the document fields
+*except* for the following cases:
 
 - The ``_id`` field is always the first field in the document.