DOCS-617 expand slightly the Isolation section in write-operations

Author: kay-kim

draft/core/write-operations.txt

@@ -284,16 +284,18 @@ and :doc:`/applications/indexes`.
 Isolation
 ---------
 
-All operations inside of a *single* MongoDB document are atomic. An
-update operation may modify more than one sub-document within a single
-MongoDB document (or record) in a single operation that will either
-succeed or fail and cannot leave the document in an in-between state.
+When a single write operation modifies multiple documents, the
+operation as a whole is not atomic, but the modification of each
+document is. The modification of a single Mongo document (or record)
+remains atomic even if the write operation modifies more than one
+sub-document *within* that document.
 
 No other operations are atomic; however, you can attempt to isolate a
-sequence of write operations from other read and write operations to
-prevent undesirable interactions. For more information see
-:doc:`Isolated write operations </reference/operator/atomic>` and
-:doc:`/tutorial/perform-two-phase-commits`.
+write operation that affects multiple documents using the :doc:`$atomic
+isolation operator </reference/operator/atomic>`.
+
+To isolate a sequence of write operations from other read and write
+operations, see :doc:`/tutorial/perform-two-phase-commits`.
 
 Architecture
 ------------