docs(vec): clarify into_boxed_slice allocation guarantee

[Walentalien]

LLM usage disclosure:
LLMs used for research and review.

Fixes #125941

r? @Mark-Simulacrum
@Mark-Simulacrum reviewed #137546 but that PR was closed due to inactivity, that review was taken into account while working on this contribution.

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[xizheyin]

If the vector is a vector of (), it won't shrink or reallocate.

View changes since the review

[xizheyin]

would be more rigor?

If `vec.len() == N` but there is remaining capacity, the allocated memory will be shrunk or reallocated, unless the type within the `vec` is `()`.

[Walentalien]

Agreed. But even more rigor would me including all zero-size types in comment not just unit type ().

Also In case when len is zero but capacity is allocated and we convert into Boxed vec deallocation of allocated capacity will happen. Consider:

let vec: Vec<u8> = Vec::with_capacity(10);
let boxed: Box<[u8; 0]> = vec.try_into().unwrap();

So new comment will be:

    /// Like [`Vec::into_boxed_slice`], this is in-place if `vec.capacity() == N`
    /// and `vec.len() == N`. If the `vec.len() == N` but there is spare capacity,
    /// the allocation will be either shrunk or reallocated, or deallocated if `N==0`.
   /// Vectors of zero-sized types have no backing allocation to modify.