[css-flexbox] Leftover from #1920 #2103

CSS Flexbox says

To provide a more reasonable default minimum size for flex items, this specification introduces a new auto value as the initial value of the min-width and min-height properties defined in CSS 2.1.

But since #1920 is is false, now this new initial value is defined in CSS Sizing.

So Flexbox should say something similar to CSS Grid:

To provide a more reasonable default minimum size for flex items, this specification defines that the auto value of min-width/min-height applies an automatic minimum size in the main axis to flex items whose overflow is visible in the main axis.

Additionally there is a problem with the bikeshed syntax,

is displayed as ‘height: auto’, which is so misleading. If you use the quote above, this paragraph should probably be deleted anyway.