Add more IEnumerable<TService> documentation

[Rick-Anderson]

See dotnet/AspNetCore.Docs#20280
Code should be moved to snippets. dotnet/AspNetCore.Docs#20281

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Document Details

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• ID: 8fd55089-b683-2202-6641-c7b61a132f14
• Version Independent ID: cc5f00d0-fb50-1229-8fa2-e66b1f45b284
• Content: Dependency injection in .NET
• Content Source: docs/core/extensions/dependency-injection.md
• Product: dotnet-core
• GitHub Login: @IEvangelist
• Microsoft Alias: dapine