[css-anchor-position][editorial] Add a note about zero-sized.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -788,6 +788,8 @@ However, the [=available space=] for the positioned element in the relevant axis
 is reduced to the size of the largest rectangle
 that is centered on the [=default anchor element=]
 and doesn't overflow the [=inset-modified containing block=].
+(Possibly being zero-sized,
+if the anchor's center is not within the [=inset-modified containing block=].)
 
 If the element is not [=absolutely positioned=],
 or does not have a [=default anchor element=],