Add a note about async threading in 5.0

[AndriySvyryd]

See dotnet/efcore#22841 (comment)

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Document Details

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• ID: a691101f-b7ea-4539-3745-6d9b0f5e317f
• Version Independent ID: 8f488a69-cedf-9c45-f977-23b3d8abd3df
• Content: Get Started with WPF - EF Core
• Content Source: entity-framework/core/get-started/wpf.md
• Product: entity-framework
• Technology: entity-framework-core
• GitHub Login: @JeremyLikness
• Microsoft Alias: jeliknes

[ajcvickers]

Note from triage: reference this from both the WPF docs (and Windows Forms when they exist) and the general async page.

[roji]

Note that this has been changed for 6.0 in dotnet/efcore#23971, so the note would no longer be valid. Give that 5.0 will go out of support soon, and that there has been very little user feedback (aside from dotnet/efcore#22841), do we still think this merits documentation?