[css-flexbox-1] be clearer about aspect ratio without definite cross size case

[fantasai]

Technically, <img style="min-width: 0; max-height: 10px;" src="100x100-green.png"> would fall under case E in https://drafts.csswg.org/css-flexbox/#algo-main-item and, hooking into CSS2 section 10, would result in a 10px flex base size. But this isn't super clear, so we might want to make it more clear; maybe by adding aspect ratio as an example in the parenthetical about cases that depend on cross size.