Add code documentation for history related stats and functions

While debugging a history issue recently, we found it hard to recollect the stat definitions for some of the history related stats. In particular, the HistoryWindowLogicalSize and HistoryLogicalSize. We should document their definitions both on lsm tree level as well as on Magma level (esp LSMTree level stats). There are some internal methods as well that give back history size, we should document them as well.

This will help in debugging issues faster and also easy to review code when changes around history code are made.