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.

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#### Document Details

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* 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**

docs/csharp/programming-guide/concepts/attributes/attributeusage.md から始め、そこにある AttributeUsage の内容を、リンクされている AttributeUsageAttribute API リファレンス記事と比較します。C# 固有ではない内容を API リファレンスに移し、C# 7.3 の段落を C# ガイド内の適切な位置に移動し、完成した記事でこれらの情報が重複したり誤った場所に配置されたりしていないことを確認します。

索引モデルが issue の本文から書いたものです。