SQL cache table still incorrect; also document the index created

This appears to be a regression in the docs. Closed issue https://github.com/dotnet/AspNetCore.Docs/issues/8383 asked about the `Id` column being shown as `NVARCHAR(900)` which led to PR https://github.com/dotnet/AspNetCore.Docs/pull/9038 to change it to 449, however the current documentation still shows 900 for some reason.

Compared to the source (below), there are other omissions like the collation declaration -- and I think the topic should also document the index the tool would create. In certain locked-down corporate scenarios, we can't just install arbitrary tools from the Internet.

https://github.com/dotnet/aspnetcore/blob/master/src/Tools/dotnet-sql-cache/src/SqlQueries.cs

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#### Document Details

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* ID: 0d01025d-86b2-bb2d-86c1-ef0695bc98e3
* Version Independent ID: 5b9a6572-3671-0d54-b2ef-83d04f8b3e96
* Content: [Distributed caching in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/performance/caching/distributed?view=aspnetcore-3.1)
* Content Source: [aspnetcore/performance/caching/distributed.md](https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/performance/caching/distributed.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-performance**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**