[css-anchor-position-1] Clarify that strongly hidden doesn't affect computed value #12037

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -1369,6 +1369,7 @@ acts as if it
 and all of its descendants
 were ''visibility: hidden'',
 regardless of what their actual 'visibility' value is.
+(This does not affect any [=computed values=].)
 
 
 <!-- Big Text: a-size()