GetOrRegisterExtension -> GetOrInitializeExtension
Add comment explaining IExtensionMessage instability

csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs

diff --git a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
index aa977db..e1d4d78 100644
--- a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
+++ b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
@@ -278,7 +278,7 @@
 

             Assert.True(message.IsInitialized());

 

-            message.GetOrRegisterExtension(TestRequired.Extensions.Multi);

+            message.GetOrInitializeExtension(TestRequired.Extensions.Multi);

 

             Assert.True(message.IsInitialized());

 

@@ -297,7 +297,7 @@
 

             Assert.True(message.IsInitialized());

 

-            message.GetOrRegisterExtension(UnittestExtensions.RepeatedBoolExtension).Add(true);

+            message.GetOrInitializeExtension(UnittestExtensions.RepeatedBoolExtension).Add(true);

 

             Assert.True(message.IsInitialized());

         }

@@ -342,7 +342,7 @@
         {

             var message = new TestAllExtensions();

             message.SetExtension(UnittestExtensions.OptionalGroupExtension, new OptionalGroup_extension { A = 10 });

-            message.GetOrRegisterExtension(UnittestExtensions.RepeatedGroupExtension).AddRange(new[]

+            message.GetOrInitializeExtension(UnittestExtensions.RepeatedGroupExtension).AddRange(new[]

             {

                 new RepeatedGroup_extension { A = 10 },

                 new RepeatedGroup_extension { A = 20 },