Rollup merge of #81706 - SkiFire13:document-binaryheap-unsafe, r=Mark-Simulacrum

Rollup merge of #81706 - SkiFire13:document-binaryheap-unsafe, r=Mark-Simulacrum

Document BinaryHeap unsafe functions

`BinaryHeap` contains some private safe functions but that are actually unsafe to call. This PR marks them `unsafe` and documents all the `unsafe` function calls inside them.

While doing this I might also have found a bug: some "SAFETY" comments in `sift_down_range` and `sift_down_to_bottom` are valid only if you assume that `child` doesn't overflow. However it may overflow if `end > isize::MAX` which can be true for ZSTs (but I think only for them). I guess the easiest fix would be to skip any sifting if `mem::size_of::<T> == 0`.

Probably conflicts with #81127 but solving the eventual merge conflict should be pretty easy.