DOCSP-35724: $in operator documentation (#372)

(cherry picked from commit 0dc9837)

Author: mcmorisi

source/fundamentals/atlas-search.txt

@@ -335,6 +335,28 @@ The search returns the following document:
 To learn more about the ``geoWithin`` operator, see the :atlas:`geoWithin </atlas-search/geoWithin>` 
 Atlas guide.
 
+In
+~~
+
+Use the ``In()`` method to search for documents with field values that match a list
+of specified values.
+
+The following example searches the ``guitars`` collection for documents that have a
+``make`` field value of either ``"Fender"`` or ``"Gibson"``.
+
+.. literalinclude:: /includes/fundamentals/code-examples/atlas-search/AtlasSearchExamples.cs
+   :start-after: start-in-search
+   :end-before: end-in-search
+   :language: csharp
+   :dedent:
+
+The search returns the following documents:
+
+.. code-block:: json
+
+   { "_id": 1, "make": "Fender", "description": "...", "establishedYear": 1946, "in_stock": true, "rating": 9 }
+   { "_id": 2, "make": "Gibson", "description": "...", "establishedYear": 1902, "in_stock": true, "rating": 8 }
+
 MoreLikeThis
 ~~~~~~~~~~~~
 

source/includes/fundamentals/code-examples/atlas-search/AtlasSearchExamples.cs

@@ -130,6 +130,16 @@ public static List<Guitar> GeoWithinSearch()
         return result;
     }
 
+    public static List<Guitar> InSearch()
+    {
+        // start-in-search
+        var result = guitarsCollection.Aggregate()
+            .Search(Builders<Guitar>.Search.In(g => g.Make, ["Fender", "Gibson"]))
+            .ToList();
+        // end-in-search
+        return result;
+    }
+
     public static List<Guitar> MoreLikeThisSearch()
     {
         // start-morelikethis-search