DOCSP 28579 restructuring project page (#7013)

* DOCSP-28579 clarifying project limitations

* DOCSP-28579 clarifying project limitations

* DOCSP-28579 tech edit

Author: ltran-mdb2

source/includes/project-stage-and-array-index.rst

@@ -1,2 +1,2 @@
 You cannot use an array index with the :pipeline:`$project` stage.
-See :ref:`example-project-array-indexes`.
+For more information, see :ref:`example-project-array-indexes`.

source/reference/operator/aggregation/project.txt

@@ -86,11 +86,11 @@ The :pipeline:`$project` specifications have the following forms:
 
        See also the :pipeline:`$unset` stage to exclude fields.
 
-Considerations
---------------
+Behavior
+--------
 
-Include Existing Fields
-~~~~~~~~~~~~~~~~~~~~~~~
+Include Fields
+~~~~~~~~~~~~~~
 
 - The ``_id`` field is, by default, included in the output documents.
   To include any other fields from the input documents in the output
@@ -101,8 +101,8 @@ Include Existing Fields
   document, :pipeline:`$project` ignores that field inclusion and
   does not add the field to the document.
 
-Suppress the ``_id`` Field
-~~~~~~~~~~~~~~~~~~~~~~~~~~
+``_id`` Field
+~~~~~~~~~~~~~
 
 By default, the ``_id`` field is included in the output documents.
 To exclude the ``_id`` field from the output documents, you
@@ -226,11 +226,25 @@ fails with the same error:
 
 .. include:: /includes/aggregation/fact-project-stage-placement.rst
 
-Restrictions
-~~~~~~~~~~~~
+Considerations
+--------------
+
+Empty Specification
+~~~~~~~~~~~~~~~~~~~
+
+MongoDB returns an error if the :pipeline:`$project` stage is passed an
+empty document. 
+
+For example, running the following pipeline produces an error: 
+
+.. code-block:: javascript
+
+   db.myCollection.aggregate( [ { 
+      $project: { } 
+   } ] )
 
-An error is returned if the :pipeline:`$project` specification is
-an empty document.
+Array Index
+~~~~~~~~~~~
 
 .. include:: /includes/project-stage-and-array-index.rst