native histoggram spec: document extrapolation below zero heuristics

[beorn7]

This is based on the discussion and insights gained in https://github.com/prometheus/prometheus/pull/16192 .

Part of https://github.com/prometheus/prometheus/issues/15976 .

@krajorama I nominate you as reviewer because you probably have the most context. But anyone else is welcome to join the discussion.

/cc @gen1321 as the author of https://github.com/prometheus/prometheus/issues/15976, which triggered the whole thought process.