This is just a special case of a broader, more general advice that I follow:
Comment on whatever would be surprising when you read the code.
When I write code, a voice in the back of my head constantly asks “will I understand this code later?”. (People who just instinctively answer ‘yes’ every time are arrogant and often wrong.) Whenever the answer is ‘not sure’, the next obvious question is “why not?”. Answering that question leads you directly to what you need to write in your comment.
Sometimes the answer is “because the reader of the code might wonder why I didn't write it another way”, and that's the special case this article covers. But sometimes the answer is “because it's not obvious how it works or why it's correct” and that clearly requires a different type of comment.
As an addition, if you first try to write the code one way, and then it doesn't work and you need a second approach, that's a really good indication that you want some kind of comment there. You were surprised while writing it, so if you'll forget that surprise in a year, you'll be surprised by reading it too.
My driving principle in the same vein is "where do I have to look when/if this doesn't behave as expected?" - if the answer is not in the docs (wiki -> package/module -> file -> class -> function / method) or is otherwise > 10 lines away, it gets an inline comment (or the docs are updated). Usually this happens when chopping up strings or during intermediate navigation steps over an odd data structure.
Comment on whatever would be surprising when you read the code.
When I write code, a voice in the back of my head constantly asks “will I understand this code later?”. (People who just instinctively answer ‘yes’ every time are arrogant and often wrong.) Whenever the answer is ‘not sure’, the next obvious question is “why not?”. Answering that question leads you directly to what you need to write in your comment.
Sometimes the answer is “because the reader of the code might wonder why I didn't write it another way”, and that's the special case this article covers. But sometimes the answer is “because it's not obvious how it works or why it's correct” and that clearly requires a different type of comment.