Small corrections

Author: papafe

src/MongoDB.Bson/BsonExtensionMethods.cs

@@ -26,7 +26,7 @@ namespace MongoDB.Bson
     public static class BsonExtensionMethods
     {
         //DOMAIN-API We should remove all the methods that do not take a serialization domain.
-        //QUESTION: Do we want to do something now about this...? It's used also internally, but it seems in most cases it's used for "default serialization", so it should be ok.
+        //QUESTION: Do we want to do something now about this...? The methods are also used internally, but it seems in most cases it's used for "default serialization" and for "ToString" methods, so it should be ok.
         /// <summary>
         /// Serializes an object to a BSON byte array.
         /// </summary>

src/MongoDB.Driver/Authentication/SaslAuthenticator.cs

@@ -257,7 +257,7 @@ private CommandWireProtocol<BsonDocument> CreateCommandProtocol(BsonDocument com
                 resultSerializer: BsonDocumentSerializer.Instance,
                 messageEncoderSettings: null,
                 serverApi: _serverApi,
-                serializationDomain: BsonSerializer.DefaultSerializationDomain); //QUESTION Is it correct to use the default serialization domain here?
+                serializationDomain: BsonSerializer.DefaultSerializationDomain);
 
         private BsonDocument CreateStartCommand(byte[] bytesToSendToServer)
         {

src/MongoDB.Driver/Core/Bindings/CoreSession.cs

@@ -454,7 +454,7 @@ private IReadOperation<BsonDocument> CreateCommitTransactionOperation(OperationC
         {
             var writeConcern = GetCommitTransactionWriteConcern(operationContext, isCommitRetry);
             var maxCommitTime = _currentTransaction.TransactionOptions.MaxCommitTime;
-            //QUESTION Is it correct we only need a default domain here?
+            //We only use BsonDocumentSerializer with the domain, so using the default one is fine.
             return new CommitTransactionOperation(_currentTransaction.RecoveryToken, writeConcern, BsonSerializer.DefaultSerializationDomain) { MaxCommitTime = maxCommitTime };
         }
 

src/MongoDB.Driver/Core/Connections/ConnectionInitializer.cs

@@ -163,7 +163,7 @@ private CommandWireProtocol<BsonDocument> CreateGetLastErrorProtocol(ServerApi s
                 resultSerializer: BsonDocumentSerializer.Instance,
                 messageEncoderSettings: null,
                 serverApi: serverApi,
-                serializationDomain: BsonSerializer.DefaultSerializationDomain); //QUESTION Is it correct to use the default serialization domain here?
+                serializationDomain: BsonSerializer.DefaultSerializationDomain);
             return getLastErrorProtocol;
         }
 

src/MongoDB.Driver/Core/Connections/HelloHelper.cs

@@ -81,7 +81,7 @@ internal static CommandWireProtocol<BsonDocument> CreateProtocol(
                 resultSerializer: BsonDocumentSerializer.Instance,
                 messageEncoderSettings: null,
                 serverApi,
-                serializationDomain: BsonSerializer.DefaultSerializationDomain); //QUESTION Is it correct to use the default serialization domain here?
+                serializationDomain: BsonSerializer.DefaultSerializationDomain); //Here and in similar cases using the default serialization domain is ok because we only serialize/deserialize BsonDocuments
         }
 
         internal static HelloResult GetResult(

src/MongoDB.Driver/Core/WireProtocol/Messages/CommandMessageSection.cs

@@ -36,9 +36,9 @@ internal abstract class CommandMessageSection
     internal abstract class Type0CommandMessageSection : CommandMessageSection
     {
         // constructors
-        public Type0CommandMessageSection(object document, IBsonSerializer documentSerializer)
+        protected Type0CommandMessageSection(object document, IBsonSerializer documentSerializer)
         {
-            Ensure.IsNotNull((object)document, nameof(document));
+            Ensure.IsNotNull(document, nameof(document));
             Document = document;
             DocumentSerializer = Ensure.IsNotNull(documentSerializer, nameof(documentSerializer));
         }
@@ -86,7 +86,7 @@ protected BatchableCommandMessageSection(
     internal abstract class Type1CommandMessageSection : BatchableCommandMessageSection
     {
         // constructors
-        public Type1CommandMessageSection(
+        protected Type1CommandMessageSection(
             string identifier,
             IBatchableSource<object> documents,
             IBsonSerializer documentSerializer,