Se'if 1 of shulchan aruch talks about if you pledged to fast a certain number of days, but that ends up including chanukah etc. or rosh chodesh.
Se'if 2 of shulchan aruch talks about pledging to davka fast on chanukah etc.

Why is it ok to fast on Rosh Chodesh etc? Why is it omitted from se'if 2?