Clarify memory semantics of the returned keys and values

[snowcall]

Hi, and thanks for the great project!

Please consider adding documentation about the memory behavior of methods that return keys and values — whether they return copies that are safe to use, or slices pointing to underlying data (like in bbolt), which must not be modified or stored.

Also, it would be great if Ascend*/Descend* methods returned an error from handleFn.

[roseduan]

Good idea, MR welcome