[csswg-drafts] [css-flexbox] Definite sizes section could use clarification (#3531)

cbiesinger has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-flexbox] Definite sizes section could use clarification ==
https://drafts.csswg.org/css-flexbox/#definite-sizes

Upon re-reading this section, I have a few suggestions for it:
* Item 3 is only referring to flex items who are stretch-aligned, I believe. Also, instead of sayings "items ... are ... definite" it should say "cross-sizes of items" are definite.
* Item 1 is not necessary, I believe. It follows automatically from item 3 / stretch handling combined with the fact that flex lines in single-line flex containers are sized to the container. So it seems this could be turned into a note / optimization opportunity (because you can avoid an extra layout pass in this case)

cc @fantasai 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3531 using your GitHub account