Warn that group joins don't work with database

[breyed]

As discussed in dotnet/efcore#17068, the examples on this page, while fine with LINQ to objects, don't work with LINQ to SQL because databases don't tend to support this type of query. Group joins kind of worked in older version of EF, but would mysteriously become slow because of silent client-side evaluation. EF Core 3 fortunately provides a runtime error, although its exception is confusing, leading to questions like this.

A great big "don't try this at home" warning on this doc page with a clear explanation of EF's limitations would save a lot of developer headaches.

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Document Details

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• ID: 0de690d2-fba4-ce90-964f-4a4eaa99e05f
• Version Independent ID: 28d797c1-3fe1-c867-fd17-dbacb6728109
• Content: Perform grouped joins (LINQ in C#)
• Content Source: docs/csharp/linq/perform-grouped-joins.md
• Product: dotnet-csharp
• Technology: csharp-linq
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

Thanks for writing this @breyed Your notes are correct. I've added this to our next sprint to address.

If you'd like to submit a PR before then, I'll watch for it.

[smitpatel]

This could be referenced https://docs.microsoft.com/en-us/ef/core/querying/complex-query-operators#groupjoin