Consolidate and revise content on Attributes in .NET

This article documents the AttributeUsage attribute. That is useful information, but I think it should be included in [the API reference article about `AttributeUsageAttribute`](https://docs.microsoft.com/en-us/dotnet/api/system.attributeusageattribute), not in the C# guide.

I think most of the information in this article is not C#-specific, so moving it to the API reference should be fine. It contains one paragraph about C# 7.3, which is not really about AttributeUsage, so that paragraph should probably be moved to another article in the C# guide.

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: d714b529-ada7-833d-dc84-59dd7f20be05
* Version Independent ID: 06a82702-a0c4-9d1c-5dbe-b70d9badfddd
* Content: [AttributeUsage (C#)](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/attributeusage)
* Content Source: [docs/csharp/programming-guide/concepts/attributes/attributeusage.md](https://github.com/dotnet/docs/blob/master/docs/csharp/programming-guide/concepts/attributes/attributeusage.md)
* Product: **dotnet-csharp**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**

Start with docs/csharp/programming-guide/concepts/attributes/attributeusage.md and compare its AttributeUsage material with the linked AttributeUsageAttribute API reference article. Move non-C#-specific content to the API reference, relocate the C# 7.3 paragraph within the C# guide, and verify the resulting articles no longer duplicate or misplace this information.

Vom Indexierungsmodell aus dem Issue-Text verfasst.