[selectors] Rephrase the selector list paragraph to provide specific definitions for 'list of complex/etc selectors'.

Author: tabatkins

selectors/Overview.bs

@@ -543,8 +543,16 @@ Structure and Terminology</h3>
 	the <a>next-sibling combinator</a> (U+002B, <code>+</code>),
 	and the <a>following-sibling combinator</a> (U+007E, <code>~</code>).
 
-	A <dfn export>selector list</dfn> is a comma-separated list of <a>selectors</a>;
-	see <a href="#grouping">Selector Lists</a>.
+	A <dfn export lt="list of simple selectors|list of compound selectors|list of complex selectors">list of simple/compound/complex selectors</dfn>
+	is a comma-separated list of
+	<a lt="simple selector">simple</a>,
+	<a lt="compound selector">compound</a>,
+	or <a>complex selectors</a>.
+	This is also called just a <dfn export lt="selector list|list of selectors">selector list</dfn>
+	when the type is either unimportant or specified in the surrounding prose;
+	if the type is important and unspecified,
+	it defaults to meaning a <a>list of complex selectors</a>.
+	See [[#grouping]] for further details on <a>selector lists</a>.
 
 	The <dfn export local-lt="subject">subject of a selector</dfn> is
 	the element(s) that selector is defined to be about: