Fix #118, Only check K projects for XML documentation errors

- ignore leftover XML files e.g. from other branches
 - tested manually in MVC repo with a bad doc file outside a K project
- align with clean target: that only removes bin directories under K projects
- approach ignores XML files in .csproj projects but no such projects in
  repos where _k-xml-docs-test.shade file is used

Address old [comment](https://github.com/aspnet/Universe/pull/101/files#r16633938) from @Eilon on PR#101
> Traditionally we use i++ and not ++i.
You could narrow it down further since the doc files share the same name as the project

```C#
var fileInfo = new FileInfo(projectFile);
var docFilesWildcard = Path.Combine(fileInfo.DirectoryName, "**", fileInfo.Directory.Name + ".xml");
foreach (var xmlFilePath in Files.Include(docFilesWildcard))
```