docs: fix param types on get_comments_number_text()

[justlevine]

This PR updates the @param types on get_comments_number_text() to correctly reflect the default false parameter.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/63268

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[SergeyBiryukov]

Thanks for the PR! Merged in r60166.