Support Sandcastle's behavior to recognize triple-slash on namespace

Triple-slash comments on the namespace itself is currently not supported. It would be great to add this, so that <summary> and <remarks> tags works, and would be presented on the API Documentation page, similar to old style MSDN pages like: http://msdn.microsoft.com/en-us/library/system.data.aspx.

Example:

/// <summary>Does not show up in .yml or .html output, but</summary>
/// <remarks>would be really nice if it did.</remarks>
namespace Boogie.On
{
}

[wekempf]

In Sandcastle, namespace documentation was generated by creating an internal dummy class named NamespaceDoc and putting the documentation on it.

/// <summary>Namespace documentation goes here.</summary>
[CompilerGenerated]
class NamespaceDoc
{
}

The CompilerGeneratedAttribute was used to ensure the class itself didn’t wind up in the documentation.

This is another Sandcastle feature that DocFX should understand/use IMHO. External documentation files would work just as well, but the more the API reference documentation lives with the code, the better, IMHO. External documentation files should be for just that… documentation external to the API reference documentation.

[bitbonk]

@cesarsouza I was hoping that migrating NamespaceDoc to overwrite files or something similar would be dead simple.

If it is not, I would rather see a (well documented) migration path that makes more sense. We should not built features into DocFX just to be compatible with SHFB.