missing doc comment for ptrAddrUnsafe and withPtrAddr

Stream: lean4

[Asei Inoue]

ptrAddrUnsafe and withPtrAddr do not have doc comments. Since they seem to be important functions, I think it would be better if they had them.

Here is a post by Mario explaining how they work:
https://leanprover.zulipchat.com/#narrow/channel/113488-general/topic/fibonacci.20function.20in.20scientific-computing-lean/near/439350217

Wouldn't this be almost directly usable as a doc comment? What do you think?

[Asei Inoue]

If it's okay to add doc comments, I am ready to submit a PR.