Update global stylesheet docblocks with custom-css parameter.

[tellthemachines]

What?

#62357 made some updates to the global styles custom CSS handling logic, but neglected to update the docblocks for both gutenberg_get_global_stylesheet and WP_Theme_JSON_Gutenberg::get_stylesheet. This PR fixes the omission.

[github-actions]

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Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

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[github-actions]

Flaky tests detected in ee0734c.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12226372102
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js