Fix doc typo in Vec::into_array and convert Arc/Box/Rc::into_arry to -> Result

[Teufelchen1]

Hello 🪼,

the first commit fixes a documentation mistake added in the recent PR #156234

The second commit addresses three function signature changes as defined by the tracking issue #148082
It converts all .into_array() -> Option<> into .into_array() -> Result<>

The third commit is a fixup for the second. The change is needed as calling unwrap() on Err(E) requires E to implement Debug. I thought Vec<T, A> where T: Debug is not an acceptable solution, so I tried to come up with my own. I am NOT confident in my unsafe {} code due to lack of expierence, please check extra carefully.

Appreciating feedback & greetings from the RustWeek,
Thanks!

[rustbot]

r? @Mark-Simulacrum

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