Clarify migration analyzer diagnostics are info-level, not visible in build output

[Copilot]

Migration documentation incorrectly stated that TUXU0001/TUNU0001/TUMS0001 diagnostics would appear as warnings in dotnet build output. These are info-level diagnostics and are not visible without explicit flags.

Changes

• Removed misleading statement about seeing diagnostics in build output from Step 3 of all migration guides
• Added optional verification step with command to check analyzer is working before applying changes:

  dotnet format analyzers --severity info --diagnostics TUXU0001 --verify-no-changes

• Applied consistently across xUnit, NUnit, and MSTest migration documentation

Users can now verify analyzer detection without confusion about why diagnostics don't appear in standard build output.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Clarify xUnit migration docs</issue_title>
<issue_description>### Problem Statement

Step 3 of the xUnit migration docs states "You should see TUXU0001 warnings in your build output for xUnit code that can be converted.". But TUXU0001 is information level and won't be in the output of dotnet build, so this step leads to confusion that the analyzer isn't working, even though it is working.

Proposed Solution

Remove this step and skip straight to applying changes, or explain that the analyzer outputs can be viewed without being applied with dotnet format analyzers --severity info --diagnostics TUXU0001 --verify-no-changes

Alternatives Considered

No response

Feature Category

Documentation

How important is this feature to you?

Nice to have - would improve my experience

Additional Context

No response

Contribution

• I'm willing to submit a pull request for this feature</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Clarify xUnit migration docs #4798

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