Remark: He did it, because he has not enough reputation to make a comment.
Questions of the form "how does equation (x) follow?" [referring to some mathematical text] are asked frequently, and make reasonably good questions on their own. The fact that the text under consideration is placed on this site makes some difference but not a lot. I would do the following:
- Post an answer to the question, as I would to other "clarify a proof" questions.
- Go back to the answer and either (a) leave a comment pointing to clarification, or (b) edit the answer.
The choice between (a) and (b) depends on the mathematical substance. If you think the proof is actually well written and the asker was confused due to insufficient background for reading the proof, a comment is enough. If you think you were sloppy/hasty when writing the answer, then edit it.