[css-align] definition of 'flex-start' and 'flex-end' are changed (relative to flexbox) to apply to out-of-flow children of flex containers

[dbaron]

The definition of flex-start and of flex-end currently say:

When used on boxes that are not children of a flex container, this value behaves as start.

There's also a subtle difference between flex items and the "children of a flex container" term used here, since flex items are in flow children of a flex container. Perhaps this should specify flex items? Or does it intend to apply to out-of-flow children of a flex container as well? If so, that's a change from flexbox that should probably be noted explicitly, since it's not obvious.

[tabatkins]

Yeah, not sure why we went with the "children of a flex container" wording; we really mean flex items, and things pretending to be flex items (such as when figuring out the static position of an an abspos child of a flex container). New wording in the commit above.