Add document to clarify #212.

Author: yfakariya

src/MsgPack/Serialization/MessagePackSerializer.cs

@@ -114,6 +114,10 @@ public void PackTo( Packer packer, object objectTree )
 /// <returns>
 ///	The deserialized object.
 /// </returns>
+/// <remarks>
+///	You must call <see cref="Unpacker.Read()"/> at least once in advance.
+///	Or, you will get a default value of the object.
+/// </remarks>
 /// <seealso cref="Capabilities" />
 public object UnpackFrom( Unpacker unpacker )
 {
@@ -229,6 +233,10 @@ public Task PackToAsync( Packer packer, object objectTree, CancellationToken can
 /// <exception cref="NotSupportedException">
 ///	The type of deserializing is not serializable even if it can be serialized.
 /// </exception>
+/// <remarks>
+///	You must call <see cref="Unpacker.Read()"/> at least once in advance.
+///	Or, you will get a default value of the object.
+/// </remarks>
 /// <seealso cref="Capabilities"/>
 public Task<object> UnpackFromAsync( Unpacker unpacker, CancellationToken cancellationToken )
 {

src/MsgPack/Serialization/MessagePackSerializer`1.cs

@@ -483,6 +483,10 @@ protected internal virtual Task PackToAsyncCore( Packer packer, T objectTree, Ca
 /// <exception cref="NotSupportedException">
 ///	<typeparamref name="T"/> is not serializable even if it can be serialized.
 /// </exception>
+/// <remarks>
+///	You must call <see cref="Unpacker.Read()"/> at least once in advance.
+///	Or, you will get a default value of <typeparamref name="T"/>.
+/// </remarks>
 /// <seealso cref="P:Capabilities"/>
 [System.Diagnostics.CodeAnalysis.SuppressMessage( "Microsoft.Design", "CA1062:ValidateArgumentsOfPublicMethods", MessageId = "0", Justification = "False positive because never reached." )]
 public new T UnpackFrom( Unpacker unpacker )
@@ -574,6 +578,10 @@ protected internal virtual T UnpackNil()
 /// <exception cref="NotSupportedException">
 ///	<typeparamref name="T"/> is not serializable even if it can be serialized.
 /// </exception>
+/// <remarks>
+///	You must call <see cref="Unpacker.Read()"/> at least once in advance.
+///	Or, you will get a default value of <typeparamref name="T"/>.
+/// </remarks>
 /// <seealso cref="P:Capabilities"/>
 public Task<T> UnpackFromAsync( Unpacker unpacker )
 {