You can use the
N() function to add a comment inline with a formula. What it actually is mean't for is to try to convert a string into a number, but if you just put words in there the output of the formula is zero.
Although come to think of it, you've probably already told this person to explain their actions and they haven't listened to you so they probably won't now! Best to keep it to yourself for explaining stuff you don't want to use a comment for.
See example below: