It's not obvious, but the Help Center actually does have (some limited) guidance about when and when not to comment. While the Help Center's Privileges pages feel a bit like an unused, forgotten card catalog in a library in the age of internet search, they are nonetheless part of the Help Center. The Comment Everywhere Privilege page has guidance on when and when not to comment:
When should I comment?
You should submit a comment if you want to:
- Request clarification from the author;
- Leave constructive criticism that guides the author in improving the post;
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).
When shouldn't I comment?
Comments are not recommended for any of the following:
- Suggesting corrections that don't fundamentally change the meaning of the post; instead, make or suggest an edit;
- Answering a question or providing an alternate solution to an existing answer; instead, post an actual answer (or edit to expand an existing one);
- Compliments which do not add new information ("+1, great answer!"); instead, up-vote it and pay it forward;
- Criticisms which do not add anything constructive ("-1, see previous comments you scallywag!"); instead, down-vote (and provide or up-vote a better answer if appropriate);
- Secondary discussion or debating a controversial point; please use chat instead;
- Discussion of community behavior or site policies; please use meta instead.
Personally, I don't think much more guidance needs to be made: it is just guidance or recommendation after all. The guidance works best when the gentle hand of the community members politely ask people to move "answerments" to actual Answers, and the commenter creates a full-fledged Answer.
I'm not sure how much additional guidance should be provided. Over-specifying the cases runs the risk of overcomplicating the situation, and creating extra cover for "but why was my comment deleted when comment X was left alone on question Y" scenarios. Too-specific rules beget rules-lawyering.
Full disclosure: I have violated all but the "-1, scallywag!" Criticism anti-pattern above, in comments, on multiple occasions. I am not immune from temptation to violate the guidance on occasion. I think it's important to note that the "policy" is more guidance or recommendation than dictum. Guidance/recommendation allows the community to steer behavior from amongst the populace, rather than be dictated to from on high.
More importantly, it allows for the exceptions to prove the rule:
Encouraging comments to first-timers. "Hi [new user], welcome to Photo.SE. This is a great answer! I especially liked [point X that you made]." I do this from time to time to hopefully provide some encouragement for a new user. This is contrary to the Compliments admonition above, but it serves a short-term use. If I were diligent and vigilant, I'd go back and remove the comment after a few days / weeks.
Fun. Sometimes, there are some real gems in the comments. Good ol' fashioned (and well-meaning) internet snark sometimes is gold. This is probably the canonical case of comment exceptions that prove the rule. Gems are the most brilliant in the absence of a bunch of shiny junk rocks. They should be allowed to shine.
Secondary discussion: I have done it many times. If it's apparent that there's going to be a bit of back-and-forth between me and another commenter, I'll actually actively churn out response comments, in order to more quickly trigger the "Please avoid extended discussions in comments; Consider moving this discussion to chat" option. Then I'll move it to chat, and flag the entire discussion chain to be deleted. Win-win.
Note that the first and last points are transitory in nature. They are short-term violation of the guidelines, followed by cleanup.