Clarify any constant can be used for _id for all

Author: asya999

source/reference/operator/aggregation/group.txt

@@ -30,8 +30,8 @@ Definition
       { $group: { _id: <expression>, <field1>: { <accumulator1> : <expression1> }, ... } }
 
    The ``_id`` field is *mandatory*; however, you can specify an
-   ``_id`` value of null to calculate accumulated values for all the
-   input documents as a whole.
+   ``_id`` value of null, or any other constant value, to calculate
+   accumulated values for all the input documents as a whole.
 
    The remaining computed fields are *optional* and computed using the
    ``<accumulator>`` operators.