Enhance group join documentation with workaround

Can you add the suggested workaround for group join mentioned on GitHub to this documentation?

https://github.com/dotnet/efcore/issues/19930#issuecomment-625443593

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#### Document Details

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* ID: fe1acf84-126c-f01d-b39a-ab077819351f
* Version Independent ID: 5d3bdd6d-ad0e-e306-5aef-0d5adea892b6
* Content: [Complex Query Operators - EF Core](https://docs.microsoft.com/en-us/ef/core/querying/complex-query-operators)
* Content Source: [entity-framework/core/querying/complex-query-operators.md](https://github.com/dotnet/EntityFramework.Docs/blob/main/entity-framework/core/querying/complex-query-operators.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @smitpatel
* Microsoft Alias: **avickers**