[css-anchor-position-1] Add issue about position-anchor initial value.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -528,6 +528,10 @@ and (by default) all [=anchor functions=] applied to this element.
 		is the box's [=default anchor element=].
 </dl>
 
+Issue(#13067): We might want to change the initial value to be slightly more magical,
+auto-choosing between ''position-anchor/none'' and ''position-anchor/auto''
+based on 'position-area' being used or not.
+
 The [=principal box=] of the [=default anchor element=]
 is the box's <dfn>default anchor box</dfn>.