[extend-rule] Give placeholder selectors universal selector specificity

See tabatkins#40

css-extend-rule/index.bs

@@ -321,10 +321,10 @@ This ensures that no element will ever directly match the styles using one,
 even by accident,
 and it can't be accidentally reused for an element directly.
 
-<a>Placeholder selectors</a> have the same specificity as <a>class selectors</a>.
+<a>Placeholder selectors</a> have the same specificity as <a>universal selectors</a>.
 
-Issue: Or should they have slightly less, so concrete classes can reliably override?
-This would mean putting a fourth number into the specificity 3-tuple.
+Issue: Or should they have greater specificity?
+Or should we put a fourth number into the specificity 3-tuple?
 
 Acknowledgements {#acks}
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