[css-anchor-position] Fix the OM to the new @position-try rule

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1601,36 +1601,28 @@ DOM Interfaces {#interfaces}
 The CSSPositionFallbackRule interface {#position-fallback-rule}
 -------------------------------------
 
-The {{CSSPositionFallbackRule}} interface represents
-the ''@position-fallback'' rule:
+The {{CSSPositionTryRule}} interface represents
+the ''@position-try'' rule:
 
 <pre class='idl' export>
 [Exposed=Window]
-interface CSSPositionFallbackRule : CSSGroupingRule {
+interface CSSPositionTryRule : CSSRule {
 	readonly attribute CSSOMString name;
+	[SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
 };
 </pre>
 
-Its <dfn attribute for=CSSPositionFallbackRule>name</dfn> attribute
-represents the name declared by the at-rule itself.
-
+Its <dfn attribute for=CSSPositionTryRule>name</dfn> attribute
+represents the name declared in the rule's prelude.
 
+Its <dfn attribute for=CSSPositionTryRule>style</dfn> attribute
+represents the styles declared in the rule's body,
+in the specified order.
+Only the [=allowed @position-try properties=] are valid in this rule.
 
-The CSSTryRule interface {#position-try-rule}
--------------------------------------
+Issue: match the concept of validity to whatever we do in similar situations.
 
-The {{CSSTryRule}} interface represents a ''@try'' block
-declared in a ''@position-fallback'' rule:
-
-<pre class='idl' export>
-[Exposed=Window]
-interface CSSTryRule : CSSRule {
-	[SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
-};
-</pre>
 
-Its <dfn attribute for=CSSTryRule>style</dfn> attribute
-represents the declarations declared by the ''@try'' block itself in the specified order.
 
 
 Security Considerations {#sec}