DOCS-14006 clarify return data types sum subtract

source/reference/operator/aggregation/subtract.txt

@@ -33,6 +33,22 @@ Definition
    first argument. For more information on expressions, see
    :ref:`aggregation-expressions`.
 
+Behavior
+--------
+
+Starting in MongoDB 4.2.12, the result will have the same type as the
+input except when it cannot be represented accurately in that type. In
+these cases:
+
+- A 32-bit integer will be converted to a 64-bit integer if the
+  result is representable as a 64-bit integer.
+
+- A 32-bit integer will be converted to a double if the result is
+  not representable as a 64-bit integer.
+
+- A 64-bit integer will be converted to double if the result is not
+  representable as a 64-bit integer.
+
 Examples
 --------
 

source/reference/operator/aggregation/sum.txt

@@ -52,6 +52,21 @@ Definition
 Behavior
 --------
 
+Result Data Type
+~~~~~~~~~~~~~~~~
+
+The result will have the same type as the input except when it cannot be
+represented accurately in that type. In these cases:
+
+- A 32-bit integer will be converted to a 64-bit integer if the
+  result is representable as a 64-bit integer.
+
+- A 32-bit integer will be converted to a double if the result is
+  not representable as a 64-bit integer.
+
+- A 64-bit integer will be converted to double if the result is not
+  representable as a 64-bit integer.
+
 Non-Numeric or Non-Existent Fields
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~