Add comment to OpenApiDocumentProvider

[martincostello]

Add comment to OpenApiDocumentProvider

Add a comment explaining what the OpenApiDocumentProvider class is for.

Description

I was looking through the code to see if it was possible for the new OpenAPI library to output YAML instead of JSON yet (context: #54598 (comment)), and at first I thought OpenApiDocumentProvider was only used by tests, so it seemed redundant.

Then I found this comment:

aspnetcore/src/OpenApi/src/Extensions/OpenApiServiceCollectionExtensions.cs

Lines 70 to 71 in 06155c0

	// Required for build-time generation
	services.AddSingleton<IDocumentProvider, OpenApiDocumentProvider>();

This PR adds a class-level summary explaining what the purpose of the internal class is, as otherwise it seems unused (it's accessed through reflection here):

aspnetcore/src/Tools/GetDocumentInsider/src/Commands/GetDocumentCommandWorker.cs

Line 27 in 06155c0

private const string DocumentService = "Microsoft.Extensions.ApiDescriptions.IDocumentProvider";

aspnetcore/src/Tools/GetDocumentInsider/src/Commands/GetDocumentCommandWorker.cs

Lines 193 to 207 in 06155c0

	Type serviceType = null;
	foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
	{
	serviceType = assembly.GetType(DocumentService, throwOnError: false);
	if (serviceType != null)
	{
	break;
	}
	}
	if (serviceType == null)
	{
	_reporter.WriteError(Resources.FormatServiceTypeNotFound(DocumentService));
	return false;
	}