DOCSP-45715: Sealed class v3.0 changes

Author: mcmorisi

source/fundamentals/serialization.txt

@@ -84,7 +84,7 @@ The following code example shows a custom ``BsonRegularExpression`` serializer:
 
 .. code-block:: csharp
 
-   class CustomRegularExpressionSerializer : SerializerBase<Regex>
+   class CustomRegularExpressionSerializer : IBsonSerializer<Regex>
    {
        public override Regex Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
      {

source/upgrade/v3.txt

@@ -203,4 +203,11 @@ Version 3.0 Breaking Changes
     property instead.
 
 - The driver removes individual cluster events from ``MongoClient.Cluster``. To listen for
-  cluster events, use `ClusterBuilder.Subscribe() <{+new-api-root+}/MongoDB.Driver/MongoDB.Driver.Core.Configuration.ClusterBuilder.Subscribe.html>`__.
+  cluster events, use `ClusterBuilder.Subscribe() <{+new-api-root+}/MongoDB.Driver/MongoDB.Driver.Core.Configuration.ClusterBuilder.Subscribe.html>`__.
+
+- All concrete serializers have been sealed. To implement a custom serializer, implement the
+  ``IBsonSerializer`` interface.
+
+- The ``MongoClient``, ``MongoDatabase``, and ``MongoCollection`` classes have been
+  sealed. We recommend using the ``IMongoClient``, ``IMongoDatabase``, and
+  ``IMongoCollection`` interfaces directly.