Improve documentation on role or user change during SignalR connection lifetime

[cincuranet]

Fixes dotnet/aspnetcore#66718.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/signalr/authn-and-authz.md	customer intent: As an ASP.NET developer, I want to use authentication and authorization in ASP.NET Core SignalR, so I can verify user access to my apps.

[Copilot]

Pull request overview

This PR updates the SignalR authentication/authorization documentation to clarify how authorization behaves when a user’s roles/claims change after a SignalR connection is established.

Changes:

• Adds a new section explaining that SignalR caches the authenticated principal for the lifetime of a connection, so role/claim updates aren’t reflected on existing connections.
• Provides guidance on mitigation options (closing connections or performing “current data” checks).
• Updates the bearer-token section to reference the new guidance.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.