[css-flexbox] Clarify automatic min size's definition if both specified size and aspect ratio are not present

[shinglyu]

https://drafts.csswg.org/css-flexbox/#min-size-auto

In the section:

In general, the automatic minimum size is the smaller of its content size and its specified size. However, if the box has an aspect ratio and no specified size, its automatic minimum size is the smaller of its content size and its transferred size.

The case which no specified size and no aspect ratio is present is not clear. People might get confused and use the old default 0.

In the previous edition, it is clear that we should use content size in that case.