Document mutation-based logic error

[apasel422]

Add this to the IntervalHeap doc:

It is a logic error for an item to be modified in such a way that the item's ordering relative to any other item, as determined by the heap's comparator, changes while it is in the heap. This is normally only possible through Cell, RefCell, global state, I/O, or unsafe code.

[Gankra]

I kinda assumed that that goes without saying but sure, why not.

[apasel422]

We added this disclaimer to the collections in std: rust-lang/rust#23342.