[selectors-5]: Provide guidance on specificity of :heading/:heading()

Author: keithamus

selectors-5/Overview.bs

@@ -162,6 +162,8 @@ Heading Structures: the heading pseudo-classes '':heading'', and '':heading()''<
 	matches an element which has a <a>heading level</a>, with respect to the
 	semantics defined by the document language (e.g. [[HTML5]]).
 
+	The [=specificity=] of '':heading'' is that of a class.
+
 	<div class="example">
 		For example, the following sheet contains a rule applying to all heading
 		elements in the current page:
@@ -178,6 +180,8 @@ Heading Structures: the heading pseudo-classes '':heading'', and '':heading()''<
 		::heading() = ::heading( <<An+B>># )
 	</pre>
 
+	The [=specificity=] of '':heading()'' is that of a class.
+
 	<div class="example">
 		The following example styles headings with levels between 1 and 3 with a
 		font-weight of 900, while headings with levels 6 onward with font-weight of