Add WithDiagnostics helper and clarify diagnostics

[jonathanvdc]

Summary

• add a helper so applications can opt into diagnostic-formatted logging once at setup time
• update the caret diagnostics example and tests to use the new helper
• improve the README and XML docs to explain what a diagnostic is and how it differs from raw rendering

Testing

• dotnet test Tests/Tests.csproj --filter "LogBehaviorTests|CaretDiagnosticTests"