chore: update migration guide

Author: baileympearson

etc/notes/CHANGES_5.0.0.md

@@ -20,12 +20,25 @@ The following is a detailed collection of the changes in the major v5 release of
 
 The `mapReduce` helper has been removed from the `Collection` class.  The `mapReduce` operation has been
 deprecated in favor of the aggregation pipeline since MongoDB server version 5.0.  It is recommended
-to migrate code that uses `Collection.mapReduce` to use the aggregation pipeline.
+to migrate code that uses `Collection.mapReduce` to use the aggregation pipeline (see [Map-Reduce to Aggregation Pipeline](https://www.mongodb.com/docs/manual/reference/map-reduce-to-aggregation-pipeline/)).
 
 If the `mapReduce` command must be used, the `Db.command()` helper can be used to run the raw
 `mapReduce` command.
 
 ```typescript
+// using the Collection.mapReduce helper in <4.x drivers
+const collection = db.collection('my-collection');
+
+await collection.mapReduce(
+  function() { emit(this.user_id, 1); },
+  function(, vals) { return 1 },
+  {
+    out: 'inline',
+    readConcern: 'majority'
+  }
+)
+
+// manually running the command using `db.command()`
 const command = {
   mapReduce: 'my-collection',
   map: 'function() { emit(this.user_id, 1); }',