can -> should

Signed-off-by: Eric Eggert <ee@w3.org>
typo, is important -> may be important

Signed-off-by: Eric Eggert <ee@w3.org>
Whitespace, oxford comma

Signed-off-by: Eric Eggert <ee@w3.org>
Better explain column and rowgroups

Signed-off-by: Eric Eggert <ee@w3.org>