DOCSP-21009 watch usage example (#307)

* updated intro

Author: kyuan-mongodb

source/usage-examples/changeStream.txt

@@ -7,9 +7,15 @@ Watch for Changes
 Open a Change Stream
 --------------------
 
-You can watch for changes to a single collection, a database, or an entire deployment in MongoDB with **Change Streams**.
-Open a change stream by calling the ``watch()`` method on a ``Collection``, ``Db``, or ``MongoClient`` object. The
-change stream emits **change event** documents when they occur.
+You can watch for changes in MongoDB using the ``watch()`` method on the
+following objects:
+
+- `Collection <{+api+}/classes/Collection.html#watch>`__
+- `Database <{+api+}/classes/Db.html#watch>`__
+- `MongoClient <{+api+}/classes/MongoClient.html#watch>`__
+
+For each object, the ``watch()`` method opens a **change stream** to
+emit **change event** documents when they occur.
 
 The ``watch()`` method optionally takes an **aggregation pipeline** which consists of an array of **aggregation stages**
 as the first parameter. The aggregation stages filter and transform the change events.