DOCSP-18204 update for group pipeline stage (#192)

Author: kanchana-mongodb

source/supported-unsupported/supported-aggregation.txt

@@ -60,6 +60,28 @@ column for details.
 
        See :ref:`query-atlas` for more information. 
 
+   * - :pipeline:`$group` 
+     - Groups input documents by the specified ``_id`` expression and 
+       for each distinct grouping, outputs a document. {+adl+} doesn't 
+       support empty string keys for the accumulator fields. 
+
+       .. example:: 
+
+          The following is **not** supported: 
+
+          .. code-block:: json 
+             :copyable: false 
+             :emphasize-lines: 4
+
+             {
+		           "$group" : {
+			           "_id" : "$representationType",
+			           "" : {
+				           "$sum" : NumberInt(1)
+			           }
+		           }
+	           }
+
    * - :pipeline:`$lookup`
      - Performs a left outer join to a collection in the same database. 
        {+adl+} provides syntax for joining collections from different