[css-align] details for multicol containers for align/justify-content don't consider column-span

[dbaron]

The details for multicol containers for how align-content and justify-content seem to assume that multicol containers only contain column boxes. However, with column-span: all this isn't really true: they contain a sequence of:

• an anonymous box (undefined!) that wraps a set of column boxes
• an box with column-span: all

(Also, I'm not sure "multicol containers" is a defined term, but there probably should be a term!)

[fantasai]

Fixed in 7da8c25, thanks!