[css-overflow-3] Resolve ambiguity

We define what 'continue' does on multicol containers, but did not
explicitly say that it applies to them, since it is a bit fuzzy whether
multicol containers (as opposed to column boxes) are block containers.

This makes it explicit instead.

Author: frivoal

css-overflow-3/Overview.bs

@@ -1069,7 +1069,7 @@ Fragmentation of Overflow: the 'continue' property</h3>
 		Name: continue
 		Value: ''continue/auto'' | ''discard''
 		Initial: ''continue/auto''
-		Applies to: [=block containers=]
+		Applies to: [=block containers=] and [=multicol containers=]
 		Inherited: no
 		Percentages: N/A
 		Computed value: specified value