DOCS-9118: clarify sortByCount expression

Author: kay-kim

source/reference/operator/aggregation/sortByCount.txt

@@ -43,11 +43,39 @@ Definition
 
       * - ``expression``
 
-        - :ref:`Expression <aggregation-expressions>` to group by. To
-          specify a :ref:`field path <agg-quick-ref-field-paths>`,
+        - :ref:`Expression <aggregation-expressions>` to group by. You
+          can specify any expression except for a document literal.
+
+          To specify a :ref:`field path <agg-quick-ref-field-paths>`,
           prefix the field name with a dollar sign ``$`` and enclose it
-          in quotes. The ``expression`` can not evaluate to an object.
-   
+          in quotes. For example, to group by the field ``employee``,
+          specify ``"$employee"`` as the expression.
+
+          .. code-block:: javascript
+
+             { $sortByCount:  "$employee" }
+
+          Although you cannot specify a document literal for the group
+          by expression, you can, however, specify a field or an
+          expression that evaluates to a document. For example, if
+          ``employee`` and ``business`` fields are document fields,
+          then the following :expression:`$mergeObjects` expression,
+          which evaluates to a document, is a valid argument to
+          :pipeline:`$sortByCounts`:
+
+          .. code-block:: javascript
+
+             { $sortByCount: { $mergeObjects: [ "$employee", "$business" ] } }
+
+          However, the following example with the document literal
+          expression is invalid:
+
+          .. code-block:: javascript
+
+             { $sortByCount: { lname: "$employee.last", fname: "$employee.first" } }
+
+   .. seealso:: :doc:`/reference/bson-type-comparison-order/`
+
 Behavior
 --------
 
@@ -97,4 +125,5 @@ order by count:
    { "_id" : "woodcut", "count" : 1 }
    { "_id" : "ukiyo-e", "count" : 1 }
    { "_id" : "satire", "count" : 1 }
-   { "_id" : "caricature", "count" : 1 }
+   { "_id" : "caricature", "count" : 1 }
+