One of the WARNING messages has to use the word "section" twice in two

One of the WARNING messages has to use the word "section" twice in two
different meanings, that cannot be helped.  But we can make this less
confusing by stating that the second instance refers to stuff like (2),
(3), and (9), and by adding the sections header the first instance
refers to, for example ERRORS or RETURN VALUES.

Source for confusion noticed by Jan Stary <hans at stare dot cz>,
better wording suggested by jmc@, tweaked by me.