Also be aware that, if you're using the Web interface you can send the author a private note. Just highlight the word or words and a bubble pops up. Pick private note and type away.

I mostly use it to let the author know about a typo they missed in proof, or, for example "I think you might want brake not break here..." (yeah, sorry, I'm that irritating guy). There's no need to clutter the responses with stuff this trivial.

The only downside is that the Web UI isn't set up to let you see the author's response unless you highlight the exact text you were commenting on, so once they've changed it the only place it appears is in the email notification Medium sends to you. IMO no biggie.