DOCS-5704 clarify who creates ObjectId

source/reference/object-id.txt

@@ -18,16 +18,17 @@ constructed using:
 - a 3-byte counter, starting with a random value.
 
 In MongoDB, documents stored in a collection require a unique
-:term:`_id` field that acts as a :term:`primary key`. Because
-ObjectIds are small, most likely unique, and fast to generate, MongoDB
-uses ObjectIds as the default value for the ``_id`` field if the
-``_id`` field is not specified. MongoDB clients should add an ``_id``
-field with a unique ObjectId. However, if a client does not add an
-``_id`` field, :program:`mongod` will add an ``_id`` field that holds
-an ObjectId.
-
-Using ObjectIds for the ``_id`` field provides the following
-additional benefits:
+:term:`_id` field that acts as a :term:`primary key`. MongoDB uses
+ObjectIds as the default value for the ``_id`` field if the ``_id``
+field is not specified; i.e. if a document does not contain a top-level
+``_id`` field, the MongoDB driver adds the ``_id`` field that holds an
+ObjectId. In addition, if the :program:`mongod` receives a document to
+insert that does not contain an ``_id`` field, :program:`mongod` will
+add the ``_id`` field that holds an ObjectId.
+
+MongoDB clients should add an ``_id`` field with a unique ObjectId.
+Using ObjectIds for the ``_id`` field provides the following additional
+benefits:
 
 - in the :program:`mongo` shell, you can access the creation time of
   the ``ObjectId``, using the :method:`~ObjectId.getTimestamp()` method.
@@ -41,8 +42,8 @@ additional benefits:
      a single system generate values, within a single second;
      ``ObjectId`` values do not represent a strict insertion order.
      Clock skew between clients can also result in non-strict ordering
-     even for values, because client drivers generate ``ObjectId``
-     values, *not* the :program:`mongod` process.
+     even for values because client drivers generate ``ObjectId``
+     values.
 
 Also consider the :doc:`/core/document/` section for related
 information on MongoDB's document orientation.