Docs: Fix missing documentation in Media and Multisite list tables

[noruzzamans]

Trac Ticket: https://core.trac.wordpress.org/ticket/64224

Why this change?

This change adds missing @global documentation tags.

This aligns the codebase with the WordPress PHP Documentation Standards, ensuring that global variables used within functions are properly documented for better static analysis and code clarity.

[github-actions]

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Props noruzzaman.

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[github-actions]

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