Add XML documentation for GlobalStatementSyntax

[Copilot]

• Understand the issue: GlobalStatementSyntax lacks XML documentation comments
• Identify the solution: Add TypeComment to Syntax.xml for GlobalStatementSyntax
• Add XML documentation to GlobalStatementSyntax in Syntax.xml
• Regenerate compiler code from Syntax.xml
• Build and verify the changes
• Review generated files to ensure documentation is present
• Address code review feedback: Consolidate into summary to avoid duplicate remarks sections
• Address review comment: Clarify that each top-level statement is wrapped in GlobalStatementSyntax

Original prompt

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This section details on the original issue you should resolve

<issue_title>Need docs about GlobalStatementSyntax</issue_title>
<issue_description>As a user, how do I find out about the purpose of GlobalStatementSyntax? Is it documented anywhere?</issue_description>

<agent_instructions>Copilot. Please add a

section for GlobalStatementSyntax in Syntax.xml.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@tmat We also need a scripting specification.

Fixes #17118

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[CyrusNajmabadi]

@copilot consider mentioning that each statement will be wrapped in a GlobalStatementSyntax at the top level.

[Copilot]

Done in commit e0a8c9a. Added clarification that each statement at the top level of a compilation unit is wrapped in a GlobalStatementSyntax node.

[CyrusNajmabadi]

@dotnet/roslyn-compiler ptal.