[css-flexbox] available space and max-height

Hi there!

I was looking at this testcase:
http://codepen.io/anon/pen/pJLwYp

Originally I was focused on the intrinsic width computation, but then
I actually realized that I can't find a spot in the flexbox spec that
says we should break lines at the max-height. It talks about the
available space, but its definition does not take max-height into
account.

Line breaking at max-height is interoperably implemented though. Did I
just miss the part of the spec where it defines that? :)

-Christian