Added documentation to generated code.

There are now summaries for:
- The Types nested class (which holds nested types)
- The file descriptor class for each proto
- The enum generated for each oneof

(Also fixed two typos.)

Generated code in next commit.

csharp/src/Google.Protobuf/CodedInputStream.cs

@@ -38,7 +38,7 @@ using System.IO;
 namespace Google.Protobuf
 {
     /// <summary>
-    /// Readings and decodes protocol message fields.
+    /// Reads and decodes protocol message fields.
     /// </summary>
     /// <remarks>
     /// <para>

csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs

@@ -252,7 +252,7 @@ namespace Google.Protobuf.Reflection
             {
                 if (fieldType != FieldType.Message)
                 {
-                    throw new InvalidOperationException("MessageType is only valid for enum fields.");
+                    throw new InvalidOperationException("MessageType is only valid for message fields.");
                 }
                 return messageType;
             }

src/google/protobuf/compiler/csharp/csharp_message.cc

@@ -169,6 +169,7 @@ void MessageGenerator::Generate(io::Printer* printer) {
     printer->Print(
       vars,
       "private object $name$_;\n"
+      "/// <summary>Enum of possibly cases for the \"$original_name$\" oneof.</summary>\n"
       "public enum $property_name$OneofCase {\n");
     printer->Indent();
     printer->Print("None = 0,\n");
@@ -202,7 +203,10 @@ void MessageGenerator::Generate(io::Printer* printer) {
     printer->Print("#region Nested types\n"
 		   "[global::System.Diagnostics.DebuggerNonUserCodeAttribute()]\n");
     WriteGeneratedCodeAttributes(printer);
-    printer->Print("public static partial class Types {\n");
+    printer->Print(
+        vars,
+        "/// <summary>Container for nested types declared in the $class_name$ message type.</summary>\n"
+        "public static partial class Types {\n");
     printer->Indent();
     for (int i = 0; i < descriptor_->enum_type_count(); i++) {
       EnumGenerator enumGenerator(descriptor_->enum_type(i));

src/google/protobuf/compiler/csharp/csharp_umbrella_class.cc

@@ -138,8 +138,10 @@ void UmbrellaClassGenerator::WriteIntroduction(io::Printer* printer) {
     "[global::System.Diagnostics.DebuggerNonUserCodeAttribute()]\n");
   WriteGeneratedCodeAttributes(printer);
   printer->Print(
+    "/// <summary>Holder for reflection information generated from $file_name$</summary>\n"
     "$access_level$ static partial class $umbrella_class_name$ {\n"
     "\n",
+    "file_name", file_->name(),
     "access_level", class_access_level(),
     "umbrella_class_name", umbrellaClassname_);
   printer->Indent();
@@ -148,12 +150,14 @@ void UmbrellaClassGenerator::WriteIntroduction(io::Printer* printer) {
 void UmbrellaClassGenerator::WriteDescriptor(io::Printer* printer) {
   printer->Print(
     "#region Descriptor\n"
+    "/// <summary>File descriptor for $file_name$</summary>\n"
     "public static pbr::FileDescriptor Descriptor {\n"
     "  get { return descriptor; }\n"
     "}\n"
     "private static pbr::FileDescriptor descriptor;\n"
     "\n"
     "static $umbrella_class_name$() {\n",
+    "file_name", file_->name(),
     "umbrella_class_name", umbrellaClassname_);
   printer->Indent();
   printer->Print(