Clarify documentation for Process.Start return value

[DanaIzadpanah]

Summary

Describe your changes here.

This PR clarifies the documentation of the Process.Start method's return value, addressing issue #10664.

• Explains how the return value depends on the UseShellExecute property.
• Provides examples of when false may be returned.

Fixes #Issue_Number (if available)

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Tagging subscribers to this area: @dotnet/area-system-diagnostics-process

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Learn Build status updates of commit 291b4d9:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics/Process.xml	✅Succeeded	View

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