Add info about PictureBox behavior changes

[adegeo]

Summary

• Updated the PictureBox.Load methods to describe new behavior starting with .NET 8.
• Cleaned up formatting issues in the sections where I pasted the information.

Fixes dotnet/docs-desktop#1722

Hide whitespace to see the actual changes.

[learn-build-service-prod]

Learn Build status updates of commit bf0fff6:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Windows.Forms/PictureBox.xml	✅Succeeded	View

For more details, please refer to the build report.

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[gewarren]

It's probably best to use an include file since it's repeated in a few places.

[learn-build-service-prod]

Learn Build status updates of commit ba644b8:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Windows.Forms/PictureBox.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 9ffec8d:

✅ Validation status: passed

File	Status	Preview URL	Details
includes/System.Windows.Forms.PictureBox/load-behavior-changes.md	✅Succeeded	View
xml/System.Windows.Forms/PictureBox.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