Add Missing XML Docs

Summary

Many API's are currently lacking XML docs or have invalid markup. As we begin to explore adding API documentation to the Prism Docs, we need to update the existing codebase with more complete XML docs so that these docs can be generated for the docs site.

HACKTOBERFEST UPDATE

Thank you for joining us for your Hacktoberfest contributions... Please take some time to familiarize yourself with this issue. If you submit PR's for a single method in a single class please expect your PR to be closed as invalid

NOTE

While updated and added XML docs should include Summaries, and information for parameters/return values, it will often be better to ensure that remarks and examples are included as this will make the generated API docs more useful. Additional information on proper doc tags can be found here.

/// <summary>
/// The GetZero method.
/// </summary>
/// <example> 
/// This sample shows how to call the <see cref="GetZero"/> method.
/// <code>
/// class TestClass 
/// {
///     static int Main() 
///     {
///         return GetZero();
///     }
/// }
/// </code>
/// </example>
public static int GetZero()
{
    return 0;
}

Start by reviewing the C# API surface for missing or invalid XML documentation, using the issue's Microsoft recommended-tags guidance and the Prism Docs generation goal as context. Done means adding valid summaries and parameter/return documentation, with remarks or examples where useful, across more than a single method in one class.