Added comments for private fields

Renamed StartMessage(name) to WriteMessageStart(name) on XmlFormatWriter as this
was intended to be an overload that did not get renamed.

src/ProtocolBuffers.Serialization/JsonFormatReader.cs

@@ -11,6 +11,7 @@ namespace Google.ProtocolBuffers.Serialization
     public class JsonFormatReader : AbstractTextReader
     {
         private readonly JsonCursor _input;
+        // The expected token that ends the current item, either ']' or '}'
         private readonly Stack<int> _stopChar;
 
         private enum ReaderState

src/ProtocolBuffers.Serialization/JsonFormatWriter.cs

@@ -168,7 +168,9 @@ namespace Google.ProtocolBuffers.Serialization
 
         #endregion
 
+        //Tracks the writer depth and the array element count at that depth.
         private readonly List<int> _counter;
+        //True if the top-level of the writer is an array as opposed to a single message.
         private bool _isArray;
 
         /// <summary>

src/ProtocolBuffers.Serialization/XmlFormatReader.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Xml;
+using System.Diagnostics;
 
 namespace Google.ProtocolBuffers.Serialization
 {
@@ -14,7 +15,9 @@ namespace Google.ProtocolBuffers.Serialization
     {
         public const string DefaultRootElementName = XmlFormatWriter.DefaultRootElementName;
         private readonly XmlReader _input;
+        // Tracks the message element for each nested message read
         private readonly Stack<ElementStackEntry> _elements;
+        // The default element name for ReadMessageStart
         private string _rootElementName;
 
         private struct ElementStackEntry
@@ -118,6 +121,7 @@ namespace Google.ProtocolBuffers.Serialization
             }
         }
 
+        [DebuggerNonUserCode]
         private static void Assert(bool cond)
         {
             if (!cond)

src/ProtocolBuffers.Serialization/XmlFormatWriter.cs

@@ -16,9 +16,11 @@ namespace Google.ProtocolBuffers.Serialization
     {
         private static readonly Encoding DefaultEncoding = new UTF8Encoding(false);
         public const string DefaultRootElementName = "root";
-        private const int NestedArrayFlag = 0x0001;
+
         private readonly XmlWriter _output;
+        // The default element name used for WriteMessageStart
         private string _rootElementName;
+        // Used to assert matching WriteMessageStart/WriteMessageEnd calls
         private int _messageOpenCount;
 
         private static XmlWriterSettings DefaultSettings(Encoding encoding)
@@ -119,7 +121,7 @@ namespace Google.ProtocolBuffers.Serialization
         /// </summary>
         public override void WriteMessageStart()
         {
-            StartMessage(_rootElementName);
+            WriteMessageStart(_rootElementName);
         }
 
         /// <summary>
@@ -127,7 +129,7 @@ namespace Google.ProtocolBuffers.Serialization
         /// After this call you can call IMessageLite.MergeTo(...) and  complete the message with 
         /// a call to WriteMessageEnd().
         /// </summary>
-        public void StartMessage(string elementName)
+        public void WriteMessageStart(string elementName)
         {
             if (TestOption(XmlWriterOptions.OutputJsonTypes))
             {
@@ -169,7 +171,7 @@ namespace Google.ProtocolBuffers.Serialization
         /// </summary>
         public void WriteMessage(string elementName, IMessageLite message)
         {
-            StartMessage(elementName);
+            WriteMessageStart(elementName);
             message.WriteTo(this);
             WriteMessageEnd();
         }