[css-flexbox] Create DoC for LC.

--HG--
extra : rebase_source : 6524c78be15d57801e9daff0f89f95c3103ebb8c
[selectors] Clarify case-insensitivity better.

--HG--
extra : rebase_source : b154773e72e483ee109794830879d30a9a3dd1b5