Improve note on HttpClientHandler.AllowAutoRedirect

[antonfirsov]

• Turn around the paragraph, emphasizing the modern behavior
• Harmonize language on .NET Core / .NET 5+ with the rest of HttpClient docs

Resolves dotnet/runtime#108588

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl

[learn-build-service-prod]

Learn Build status updates of commit 94dd98e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/HttpClientHandler.xml	✅Succeeded	View

For more details, please refer to the build report.

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[learn-build-service-prod]

Learn Build status updates of commit 96b37fd:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/HttpClientHandler.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel