[css-anchor-positioning-1] Clarify that an element can anchor to another within the same skipped contents (w3c#10583)

* Update Overview.bs

* rephrase, add note

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Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: chrishtr

css-anchor-position-1/Overview.bs

@@ -386,7 +386,12 @@ in anchor positioning.
 	* |el| is either an [=element=]
 		or a [=part-like pseudo-element=].
 
-	* |el| is not in the [=skipped contents=] of another element.
+	* If |el| is in the [=skipped contents=] of another element,
+		then |query el| is in the [=skipped contents=] of that same element.
+
+		Note: In other words, |query el| can anchor to |el| 
+		if they're both in the same skipped "leaf",
+		but it can't anchor "across" leafs.
 
 	* |el| is in scope for |query el|,
 		per the effects of 'anchor-scope' on |query el|