[css-flexbox] clarify align-content for multi-line containers with one line

[markbrown]

In https://drafts.csswg.org/css-flexbox-1/#align-content-property, align-content:space-between is not well defined if there happens to be only one line in the multi-line container.

Should it behave as flex-start, similarly to justify-content:space-between when there is only one item?

[fantasai]

Fixed, thanks. :)