Document appcontext switch for catch all routing fix

[rynowak]

See: dotnet/aspnetcore#21114

This is a relatively impactful bug that we're fixing with an opt-in switch in an upcoming patch. Once this is gone out in servicing we should document the switch prominently.

This should go in the 3.1 version of the main routing doc where catch all routes are discussed, as well as in the 2.2->3.X migration docs. Users porting an app from 2.1->3.1 have a high potential to be broken if they make use of catch all routes in some specialized ways.

This bug is already fixed in 5.0, so we don't need to document the workaround there.

The fix looks like:

AppContext.SetSwitch("Microsoft.AspNetCore.Routing.UseCorrectCatchAllBehavior", true);

Put this in Program.Main as the first statement.

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Document Details

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• ID: 32374077-e02e-d6a6-1cea-05e58447b7cd
• Version Independent ID: 86aeab13-1c25-9fc1-4e56-0390cb40c242
• Content: Routing in ASP.NET Core
• Content Source: aspnetcore/fundamentals/routing.md
• Product: aspnet-core
• Technology: aspnetcore-fundamentals
• GitHub Login: @Rick-Anderson
• Microsoft Alias: riande

[Rick-Anderson]

Add this to Catch-all section in Route template reference
and Migrate from ASP.NET Core 2.2 to 3.0

@rynowak Next time Can you create an issue from the doc page? That will connect the doc to the issue. Go to the bottom of the page and select This page .