[rustdoc] Inconsistency re-exporting items without #[doc(hidden)] from private modules

[xizheyin]

Description

As discussion in #137342 , After hide the item whose source is hidden,
I experimented with different cases and I found another inconsistency, No.7. I think we should hide the case in No.7 for consistency? If someone wants to inline it, just use #[doc(inline)].

In the meantime, the documentation needs to be updated to be more clear.

cc @GuillaumeGomez @lolbinarycat

No	source in private module?	source is hidden?	reexport chain has hidden?（exclude source）	visible?	inline?	Note
1	0	0	0	1	no
2	0	0	1	0		fix in #137534
3	0	1	1	0		fix in #137534
4	0	1	0	0		fix in #137534
5	1	1	0	0
6	1	1	1	0
7	1	0	1	1	1	Bug?
8	1	0	0	1	1

Source code

No1

// Source in a public module
pub mod public_mod {
    pub struct PublicItem;
}

// Simple reexport
pub use public_mod::PublicItem;

// Result: PublicItem is NOT inlined, just shown as reexport

No2

// Source in a public module
pub mod public_mod {
    pub struct PublicItem;
}

// Hidden reexport
#[doc(hidden)]
pub use public_mod::PublicItem as HiddenItem;

// Reexport of the hidden item
pub use self::HiddenItem as ReexportedItem;

// Result: ReexportedItem is inlined from PublicItem

No3

// Source in a public module but hidden
pub mod public_mod {
    #[doc(hidden)]
    pub struct HiddenSource;
}

// Reexport with hidden attribute
#[doc(hidden)]
pub use public_mod::HiddenSource as MiddleItem;

// Reexport of the hidden item
pub use self::MiddleItem as FinalItem;

// Result: FinalItem is inlined from HiddenSource, but without HiddenSource docs

No4

// Source in a public module but hidden
pub mod public_mod {
    #[doc(hidden)]
    pub struct HiddenSource;
}

// Direct reexport of hidden item
pub use public_mod::HiddenSource as ReexportedItem;

// Result: ReexportedItem is inlined from HiddenSource

No5

// Source in a private module and hidden
mod private_mod {
    #[doc(hidden)]
    pub struct HiddenPrivateItem;
}

// Reexport without hidden
pub use private_mod::HiddenPrivateItem as ReexportedItem;

// Result: ReexportedItem is inlined from HiddenPrivateItem

No6

// Source in a private module and hidden
mod private_mod {
    #[doc(hidden)]
    pub struct HiddenPrivateItem;
}

// Hidden reexport
#[doc(hidden)]
pub use private_mod::HiddenPrivateItem as MiddleItem;

// Final reexport
pub use self::MiddleItem as FinalItem;

// Result: FinalItem is inlined from HiddenPrivateItem

No7

// Source in a private module
mod private_mod {
    pub struct PrivateModItem;
}

// Hidden intermediate reexport
#[doc(hidden)]
pub use private_mod::PrivateModItem as HiddenReexport;

// Final reexport
pub use self::HiddenReexport as FinalItem;

// Result: FinalItem is inlined from PrivateModItem

No8

// Source in a private module
mod private_mod {
    pub struct PrivateModItem;
}

// Direct reexport
pub use private_mod::PrivateModItem;

// Result: PrivateModItem is inlined in documentation