Docs: Systematically add missing @global annotations for global variables

[noruzzamans]

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

Why this change?

This change adds missing @global documentation tags to functions in
src/wp-admin/includes/misc.php
src/wp-admin/includes/template.php
src/wp-admin/install.php

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.

Files Changed:
src/wp-admin/includes/misc.php
src/wp-admin/includes/template.php
src/wp-admin/install.php

[github-actions]

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Props noruzzaman, huzaifaalmesbah.

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[github-actions]

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[huzaifaalmesbah]

@noruzzamans Looks good. i have suggest minor changes.