Support documentation comments

[adamconnelly]

Thrift supports adding javadoc style comments to Thrift IDL files, and including the documentation in the generated code. If we take the following Thrift definition:

/**
 * Describes the different types of user.
 */
enum UserType {
  /**
   * Represents an administrator.
   */
  Administrator
}

It might end up generating the following code:

/// <summary>
/// Describes the different types of user.
/// </summary>
public enum UserType
{
    /// <summary>
    /// Represents an administrator.
    /// </summary>
    Administrator
}

Things to consider:

• I think the Thrift compiler only supports adding basic summaries to elements, rather than documenting params, exceptions, return types, etc.
• It only supports javadoc comments. We could also support C# XML comments to make it more familiar to C# developers.