Adding a comment seems inappropriate, as the comment does not add anything significant.
A comment inviting the author to include information in the question body can be helpful. If the suggested action is taken, the quality of the question is improved, the number of comments is reduced, and it can help new people learn how to use the Q & A format effectively.
This option does result in some delay and extra effort, since the author must see your comment and manage to edit the post (some new users don't quite have the hang of editing posts).
Either way, once the edits are made, and the comments are integrated into the post, someone needs to flag the redundant comments as obsolete so a moderator can clean them up.
Editing the question to include the comment seems inappropriate, as OP might have phrased it differently.
This is even better, when done well. The author of the post will be notified of the edit, so they can improve it further if necessary (or even roll it back if you really mess up).
You may also leave a comment explaining what you did if you wish. It is helpful to flag any comments made obsolete by your actions as well.
This option provides an immediate benefit, while making a minimum of work for others.
It is even more helpful in cases when the OP has posted with an unregistered account and then adds comments from another account (which happens quite often). We can of course wait until they fix/merge their accounts but having all the information in the question as soon as possible helps avoiding irrelevant answers - from people who haven't read all the comments.
From Why can people edit my posts? How does editing work? in the Help Centre: (emphasis added)
When should I edit posts?
Any time you see a post that needs improvement and are inclined to suggest an edit, you are welcome to do so. The original author of a question or answer may always edit their own post, regardless of reputation level.
Edits are expected to be substantial and to leave the post better than you found it. Common reasons for edits include:
- To fix grammar and spelling mistakes
- To clarify the meaning of the post (without changing that meaning)
- To include additional information only found in comments, so all of the information relevant to the post is contained in one place
- To correct minor mistakes or add updates as the post ages
- To add related resources or hyperlinks
Nevertheless, an edit is still fundamentally a suggestion for improvement, even after the Edit Questions and Answers privilege has been earned.
If there is resistance to your suggestion, even unwarranted and unjustifiable resistance -- just let go and move on. In other words: Do not attempt to force someone to accept an edit. Edits are good; fighting is bad. Ultimately there are moderators to adjudicate in the rare cases where an author's preference might need to be overridden for the benefit of the wider community.
The main goals here are to answer people's questions, and to improve the final quality of the lasting Q & A content (which does not include comments).
