rustdoc: hide `#[repr(transparent)]` if it isn't part of the public ABI
Fixes#90435.
This hides `#[repr(transparent)]` when the non-1-ZST field the struct is "transparent" over is private.
CC `@RalfJung`
Tentatively nominating it for the release notes, feel free to remove the nomination.
`@rustbot` label needs-fcp relnotes A-rustdoc-ui