[css-anchor-position-1] Make position-anchor a reset-only sub-property of position. w3c#10321

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -414,6 +414,7 @@ for all [=anchor functions=] on the element,
 allowing multiple elements to use the same set of [=anchor functions=]
 (and [=position options lists=]!)
 while changing which [=anchor element=] each is referring to.
+'position-anchor' is a [=reset-only sub-property=] of 'position'.
 
 The [=target anchor element=] selected by the [=default anchor specifier=]
 (if one exists)