DOCSP-17642 add cursor definition (#142)

Author: biniona-mongodb

source/fundamentals/crud/read-operations/cursor.txt

@@ -13,13 +13,13 @@ Access Data From a Cursor
 Overview
 --------
 
-In this guide, you can learn how to access data using a cursor with the
+In this guide, you can learn how to access data using a **cursor** with the
 MongoDB Java driver. 
 
-Read operations that return multiple documents do not immediately return
-all values matching the query. Because a query can potentially match
-a large number of documents, we need to be able to access or store the
-matched documents. 
+A cursor is a mechanism that allows an application to iterate over database
+results while only holding a subset of them in memory at a given time. The
+driver uses cursors in read operations that match multiple documents to return
+matched documents in batches as opposed to returning them all at once.
 
 This page uses an initiating method, ``find()`` to show how to access
 data from a :java-docs:`FindIterable