[css-nesting-1] Clarify that pseudo-elements can't yet be reprepresented by &. w3c#7503 w3c#7433

Author: tabatkins

css-nesting-1/Overview.bs

@@ -709,6 +709,38 @@ Nesting Selector: the ''&'' selector {#nest-selector}
 		</pre>
 	</div>
 
+	The [=nesting selector=] cannot represent pseudo-elements
+	(identical to the behavior of the '':is()'' pseudo-class).
+
+	<div class=example>
+		For example, in the following style rule:
+
+		<pre class=lang-css>
+		.foo, .foo::before, .foo::after {
+			color: red;
+
+			&:hover { color: blue; }
+		}
+		</pre>
+
+		the ''&'' only represents the elements matched by ''.foo'';
+		in other words, it's equivalent to:
+
+		<pre class=lang-css>
+		.foo, .foo::before, .foo::after {
+			color: red;
+		}
+		.foo:hover {
+			color: blue;
+		}
+		</pre>
+	</div>
+
+	Issue: We'd like to relax this restriction,
+	but need to do so simultaneously for both '':is()'' and ''&'',
+	since they're intentionally built on the same underlying mechanisms.
+	(<a href="https://github.com/w3c/csswg-drafts/issues/7433">Issue 7433</a>)
+
 	The <a>specificity</a> of the <a>nesting selector</a>
 	is equal to the largest specificity among the complex selectors
 	in the parent style rule's selector list