Fix ValueFactory return signature

[simonz-bq]

The return signatures of the factory constructors should've been updated to return IIonValue when the tree was collapsed.

[tgregg]

Good change. In the documentation for each of the interface methods please add a note that specifies which interface methods are implemented by the returned IIonValue instance. For example:

        /// Constructs a new {@code null.blob} instance.
        /// </summary>
        /// <returns>A new {@code null.blob} instance, which implements the IIonBlob interface methods.</returns>
        IIonValue NewNullBlob();

Also, this is unrelated to this change, but it doesn't look like the {@code ...} tags are valid in C# documentation comments. We might want to fix those while we're at it. Looks like <c></c> is the alternative. It would also be good for references to interfaces to be links, but I haven't looked deeply enough at the documentation comments documentation to know the best way to do that yet.

[tgregg]

I will merge this once the incomplete tag is fixed.