DOCSP-21319 BACKPORT (#1322)

Author: Dave Cuthbert

source/reference/operator/aggregation/group.txt

@@ -15,11 +15,19 @@ Definition
 
 .. pipeline:: $group
 
-   Groups input documents by the specified ``_id`` expression and for
-   each distinct grouping, outputs a document. The ``_id`` field of each
-   output document contains the unique group by value.  The output
-   documents can also contain computed fields that hold the values of
-   some :ref:`accumulator expression <accumulators-group>`.
+   The ``$group`` stage separates documents into groups according to a
+   "group key". The output is one document for each unique group key.
+   
+   A group key is often a field, or group of fields. The group key can
+   also be the result of an expression. Use the ``_id`` field in the
+   ``$group`` pipeline stage to set the group key. See below for
+   :ref:`usage examples <ex-agg-group-stage>`.
+
+   In the ``$group`` stage output, the ``_id`` field is set to the
+   group key for that document.
+   
+   The output documents can also contain additional fields that are
+   set using :ref:`accumulator expressions <accumulators-group>`.
 
    .. note::
 
@@ -32,7 +40,7 @@ Definition
       { 
         $group: 
           { 
-            _id: <expression>, // Group By Expression
+            _id: <expression>, // Group key
             <field1>: { <accumulator1> : <expression1> }, 
             ... 
           } 
@@ -46,10 +54,12 @@ Definition
         - Description
 
       * - ``_id``
-        - *Required.* If you specify an ``_id`` value of null, or any
-          other constant value, the ``$group`` stage calculates
-          accumulated values for all the input documents as a whole.
-          :ref:`See example of Group by Null <null-example>`.
+        - *Required.* The ``_id`` expression specifies the group key.
+          If you specify an ``_id`` value of null, or any other
+          constant value, the ``$group`` stage returns a single
+          document that aggregates values across all of the input
+          documents. :ref:`See the Group by Null example
+          <null-example>`.
 
       * - ``field``
         - *Optional.* Computed using the
@@ -119,6 +129,8 @@ the first document of each group.
         }
       ])
 
+.. _ex-agg-group-stage:
+
 Examples
 --------