DOCS-664 review edits

Author: Bob Grabar

source/faq/developers.txt

@@ -162,14 +162,17 @@ storage efficiently in such situations.
 
 - Use the ``_id`` field explicitly.
 
-  MongoDB automatically adds an :term:`ObjectId` to each document and sets it to
+  MongoDB clients automatically add an :term:`ObjectId` to each document and set it to
   a unique value. Additionally this field in indexed. For tiny documents
-  this takes up significant space. To optimize, use the ``_id``
-  explicitly. Take a field that is unique for the collection and store
-  its values in ``_id``. By doing so, you have explicitly provided IDs.
-  This eliminates the creation of a separate ``_id`` field. If your
-  previously separate field was indexed, this eliminates an extra index,
-  too.
+  this takes up significant space.
+
+  To optimize, users can provide a value for the ``_id`` field
+  explicitly. The goal is to leverage the ``_id`` field to store a value
+  that would have occupied space in another portion of the document. The
+  only requirement for the ``_id`` field value is that it must serve as
+  a primary key for documents in the collection by uniquely identifying
+  them. If the field's value is not unique, then it cannot serve as a
+  primary key as there would be collisions in the namespace.
 
 - Use shorter field names. This is recommended only in certain situations.