DOCS-1185 clarify _id and projection doc

source/applications/indexes.txt

@@ -136,33 +136,42 @@ located sequentially on disk.
 
 MongoDB automatically uses an index that covers a query when possible.
 To ensure that a query is covered, create an index that includes all
-the fields listed in the query result and the :ref:`query document
-<read-operations-query-document>`. This means that if the index does
-**not** include the ``_id`` field, the :term:`projection` document,
-which specifies the fields MongoDB returns, must explicitly exclude the
-``_id`` field from the result set.
+the fields listed in the :ref:`query document
+<read-operations-query-document>` and in the query result. By default,
+the ``_id`` field is included in the query result. So, if the index
+does **not** include the ``_id`` field, then for the index to be able
+to cover the query, specify the exclusion of the ``_id`` field (i.e.
+``_id: 0``) in the :ref:`projection <projection>` document.
 
-Consider the following example where the collection ``user`` has
-an index on the fields ``user`` and ``status``:
+.. example::
 
-.. code-block:: javascript
+   A collection ``users`` has the following index on the fields
+   ``user`` and ``status``:
 
-   { status: 1, user: 1 }
+   .. code-block:: javascript
 
-Then, the following query which queries on the ``status`` field and
-returns only the ``user`` field is covered:
+      { status: 1, user: 1 }
 
-.. code-block:: javascript
+   Then, the following query which queries on the ``status`` field and
+   returns only the ``user`` field is covered:
 
-   db.users.find( { status: "A" }, { user: 1, _id: 0 } )
+   .. code-block:: javascript
 
-However, the following query that uses the index to match documents is
-**not** covered by the index because it returns both the ``user`` field
-**and** the ``_id`` field:
+      db.users.find( { status: "A" }, { user: 1, _id: 0 } )
 
-.. code-block:: javascript
+   In the operation, the projection document explicitly specifies
+   ``_id: 0`` to exclude the ``_id`` field from the result since the
+   index is only on the ``status`` and the ``user`` fields.
+
+   If the projection document does not specify the exclusion of the
+   ``_id`` field, the query returns the ``_id`` field. The following
+   query is **not** covered by the index on the ``status`` and the
+   ``user`` fields because with the projection document ``{ user: 1
+   }``, the query returns both the ``user`` field and the ``_id`` field:
+
+   .. code-block:: javascript
 
-   db.users.find( { status: "A" }, { user: 1 } )
+      db.users.find( { status: "A" }, { user: 1 } )
 
 An index **cannot** cover a query if:
 

source/core/read-operations.txt

@@ -362,8 +362,10 @@ The second argument to the :method:`find() <db.collection.find()>`
 method is a projection, and it takes the form of a :term:`document` with
 a list of fields for inclusion or exclusion from the result set. You
 can either specify the fields to include (e.g. ``{ field: 1 }``) or specify the
-fields to exclude (e.g. ``{ field: 0 }``). The ``_id`` field is implicitly
-included, unless explicitly excluded.
+fields to exclude (e.g. ``{ field: 0 }``). The ``_id`` field is, by
+default, included in the result set. To exclude the ``_id`` field from
+the result set, you need to specify in the projection document the
+exclusion of the ``_id`` field (i.e. ``{ _id: 0 }``).
 
 .. note::
 
@@ -393,8 +395,8 @@ Consider the following projection specifications in :method:`find()
 
      db.inventory.find( { type: 'food' }, { item: 1, qty: 1 } )
 
-- You can remove the ``_id`` field by excluding it from the
-  projection, as in the following example:
+- You can remove the ``_id`` field from the results by specifying its
+  exclusion in the projection, as in the following example:
 
   .. code-block:: javascript