Conflicts between ExcelFunction and ExcelFunctionDoc attribute

Hi

When I include both ExcelFunction and ExcelFunctionDoc function attributes the doc gen will fail

```
Unhandled Exception: System.Reflection.AmbiguousMatchException: Multiple custom attributes of the same type found.
1> at System.Attribute.GetCustomAttribute(MemberInfo element, Type attributeType, Boolean inherit)
1> at ExcelDnaDoc.Utility.ExcelDnaHelper.IsValidFunction(MethodInfo method, Boolean explicitExports)
1> at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
1> at System.Linq.Enumerable.d__17`2.MoveNext()
1> at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
1> at System.Linq.Lookup`2.Create[TSource](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
1> at System.Linq.GroupedEnumerable`3.GetEnumerator()
1> at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
1> at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
1> at System.Linq.OrderedEnumerable`1.d__1.MoveNext()
1> at ExcelDna.Documentation.Models.AddInModel.d__13.MoveNext()
1> at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
1> at CompiledRazorTemplates.Dynamic.aafaffbb.Execute()
1> at RazorEngine.Templating.TemplateBase.RazorEngine.Templating.ITemplate.Run(ExecuteContext context)
1> at RazorEngine.Razor.Parse[T](String razorTemplate, T model)
1> at ExcelDnaDoc.Templates.ViewBase`1.Publish()
1> at ExcelDnaDoc.HtmlHelp.Create(String dnaPath, String helpSubfolder, Boolean excludeHidden, Boolean skipCompile, Boolean runAsync)
1> at ExcelDnaDoc.Program.Main(String[] args)
```

I can see ExplicitRegistration and SuppressOverwriteError have been included in both attributes therefore this error seems to be logical outcome.

Can you please confirm this is a bug and advise?

Thank you very much.

Casbby