[css-tables] 2.2.1 Add in caveat for flex and grid items

[gregwhitworth]

According to this resolution from the Paris F2F, the fixup is not as clean in a grid or flex context. We need to add this to section 2.2.1

[tabatkins]

Flex and Grid both explicitly call out that box-tree fixup happens after blockification.

[gregwhitworth]

Cool, but we should add a note for this so that anyone addressing tables that may not be aware of the flex/grid specific changes knows to check these.