Clarify StartupObject documentation: when it's needed and qualification requirements

[Copilot]

The documentation for the StartupObject compiler option was misleading about when the option is actually needed and how qualification works.

Issues Fixed

1. Misleading requirement description: The documentation implied StartupObject was only used when there are multiple Main methods, but didn't clarify that it's actually ignored when there's exactly one acceptable Main method.

2. Unclear qualification requirements: The documentation stated the class name "must be fully qualified" without explaining that this only applies when the option is actually required (multiple Main methods exist).

3. Missing global namespace scenario: No explanation of how fully qualified names work when the class is in the global namespace.

Changes Made

• Clarified that StartupObject is only needed "when your compilation includes more than one type with a Main method"
• Added explicit statement: "When there's exactly one acceptable Main method in the project, this option is ignored"
• Updated qualification requirement to: "When this option is required (multiple Main methods exist), the provided class name must be fully qualified"
• Added explanation for global namespace: "If the class is in the global namespace, the fully qualified name is the same as the simple class name"

The documentation now accurately reflects the compiler behavior as clarified by @BillWagner in the issue comments.

Fixes #33547.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-options/advanced.md	docs/csharp/language-reference/compiler-options/advanced

[BillWagner]

This is ready for final review.