Improve Assembly docs

[jkotas]

Summary

• Note that Assembly.Load creates assembly in isolated load context
• Move .NET-neutral content to be first in the remarks
• Generalize description of BadImageFormatException (delete explicit .NET Framework 2.0 references)
• Delete .NET Framework 1.x content

Follow up on dotnet/runtime#99203

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

cc @agocke @elinor-fung

[learn-build-service-prod]

Learn Build status updates of commit b9aa9a1:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Reflection/Assembly.xml	✅Succeeded	View
xml/System/AppDomain.xml	✅Succeeded	View
xml/System/Type.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

[learn-build-service-prod]

Learn Build status updates of commit e393abf:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Reflection/Assembly.xml	✅Succeeded	View
xml/System/AppDomain.xml	✅Succeeded	View
xml/System/Type.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

[learn-build-service-prod]

Learn Build status updates of commit c985729:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Reflection/Assembly.xml	✅Succeeded	View
xml/System/AppDomain.xml	✅Succeeded	View
xml/System/Type.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