Document that `Property` creates indexer properties on property bag entity types

The example for [Property bag entity types](https://docs.microsoft.com/en-us/ef/core/modeling/shadow-properties#property-bag-entity-types) configures properties using `Property`. Shouldn't that be `IndexerProperty` instead in accordance with the description

> Entity types that contain only indexer properties are known as property bag entity types.

as well as the [Shared-type entity types and property bags](https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-5.0/whatsnew#shared-type-entity-types-and-property-bags) example from "What's New in EF Core 5.0"?

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* ID: 550a3c36-7f1d-efa7-3ed8-769720168da4
* Version Independent ID: 627c4301-e00d-826e-d545-3a244305dec9
* Content: [Shadow and Indexer Properties - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/shadow-properties#property-bag-entity-types)
* Content Source: [entity-framework/core/modeling/shadow-properties.md](https://github.com/dotnet/EntityFramework.Docs/blob/main/entity-framework/core/modeling/shadow-properties.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @AndriySvyryd
* Microsoft Alias: **avickers**