TL;DR: Yes, you should edit this information into the post.
trespassing on someone else’s questions or answers.
That's not the case here though. People will leave comments to add details but forget to edit their question because they don't know that this is what we'd like to see on Stack Exchange.*
New users are still often treating our sites like forums, where everything is based on chronologically sorted threads—so they'll post comments or even answer their question with further info, rather than editing their post.
And even when they do, many of them litter their post with EDIT marks, which are absolutely irrelevant since the revision of a post is stored anyway and we usually don't need to know when which part was added.
So by all means, if you see the OP clarifying their post in a comment instead of the actual question or answer, then go ahead and edit that in. Make sure to specifically state in your edit reason that you copied the OP's comment. Otherwise your edits may be rejected as too radical.
And let's step back for a moment and look at the broader issue (or cause of your concerns). This site is not about the users—it's primarily about the content. We're curating content for the sake of delivering quality information to visitors who have similar problems.
Of course, you should not be stepping on someone's toes by changing the meaning of a post or insisting on miniscule but unnecessary changes (e.g. British vs. American English). But if you can improve a post—in terms of formatting, spelling, grammar or even content—then you should by all means do it.
* Well, in fact the message under closed posts is a little misleading because it says "If this question can be reworded […] please […] leave a comment." This is certainly not going to help the OP get their question reopened unless someone sees the comment, edits the post themselves, and then votes to reopen.