Where should this comment live?
I strongly recommend that Pull Request authors perform a self-review before requesting time from their peers. Trying to put yourself in the shoes of someone who hasn't gone through the involved process of making a code change often results in additional discoveries and improvements.
When I review my own Pull Requests, I often find that code I thought was obvious as I was writing it, probably isn't so obvious with time or distance. One subtle failure mode I've experienced in addressing this is leaving explanatory comments in GitHub's review tooling. For example, this week I wrote an explanatory review comment to the effect of:
This code change exists because in some cases the caller wants to use a raw string, rather than an object that is marshalled to JSON.
While there's an argument to be made that a better design might obviate the need for this comment, it's only intended to be illustrative of a common scenario. The key recognition here is that this comment is only useful to the reviewer, and to anyone who happens across it in future. It would be much more valuable to live in the code itself.
So if you're doing a self-review and catch yourself writing explanatory notes for your reviewers, ask: "Where should this comment live?"