cite "The restriction about appearing only in the result list or HAVING clause applies with respect to the query level that the aggregate belongs to."
May be "the restriction above"?
Or, "The restriction that variables must appear only in the result list or HAVING clause applies to the query level to which the aggregate expression belongs." (I would consider emphasizing the word belongs).
Really, just adding the word variables - "The restriction about variables appearing only" - would make it technically correct.
Your proposal of only changing "about" to "above" makes it grammatically wrong and fails to fix the underlying problem. "about" it is right word but the sentence fails to provide "what", what must only appear in the result list or having clause?
A meaningful example might help too - I'm having trouble imaging when one would choose to write a subquery whose effect would end up being treated as part of the main query.
