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

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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