Document that Layout::from_size_align does not allow align=0
Document that Layout::from_size_align does not allow align=0

This was already implied since zero is not a power of two, but maybe
worth pointing out.

aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust