What do you do more of, read PR commit messages or write them? Like almost everyone, I read a lot more PR messages than I write. And even more than the code it represents, which represents your intent to the compiler/interpreter, PR messages are about expressing your intent to the reader. And like good code, the purpose of your PR message is to meet a business need. In this case, the business need is to represent to the reader (which is often future you) what and why the PR exists in the first place.
So how do you achieve that goal? By remembering it while you're writing the message. And it starts with the title. It needs to be short and imperative. It should say what the PR does. And it probably shouldn't have the word and in it. In general, if it's a compound statement you should have split the change into multiple PRs, each with a single focus.
Second, no more than three paragraphs that explain, at a high level, what the PR does, why it's needed, and why it does it that way. It's not a design doc, but you can link to one if needed. It's not a trade study, but it should capture the reasoning. This is the place where you would touch on what you didn't do, and why you didn't do it. If there's a requirements doc, work item, task, or bug that the PR is in service of, this is where you link to it.
Third, describe how you know it works. Again in a paragraph or two, not a full test matrix, explain how you tested it. It could be unit tests, integration tests, A/B tests, or something else. And if there's something you didn't test, note what it is and why you didn't test it.
Finally, remember that we do squashed PRs, so all of the individual commit messages you write prior to submitting the PR won't be seen after your PR lands. I don't put nearly as much effort into them as I do the final PR commit message. I view them as short term notes to myself to help me write the final PM message. For the interim commits short is good, and unless there's something novel in a commit then one or two lines is enough.
So what do you think is important in a PR comment?