docs: add gindexIterator comment

[MCJOHN974]

Motivation

I'm a user of persistent-merkle-tree. I was reading the code to understand how it works internally, and spent quite a lot of time to understand what this function does. Hope this comment will save somebody a bit of time and not deviates comments style in the project.

Description

Just a comment which describes gindexIterator function

Steps to test or reproduce

Not relevant