[css-pseudo-4][editorial] Added an explanatory paragraph explaining why there's no Pseudo 3. w3c#13125

Author: tabatkins

css-pseudo-4/Overview.bs

@@ -59,6 +59,13 @@ spec:css-backgrounds-3; type:property; text:box-shadow
   For example, ''::marker::before'' is not allowed;
   but ''::before::marker'' is.
 
+  Note: Level 4 is the first version of the CSS Pseudo specification
+  under this name.
+  It was branched from the Selectors 3 [[SELECTORS-3]] specification,
+  and thus started at level 4,
+  with the remainder of Selectors 3 becoming Selectors 4 [[SELECTORS-4]].
+
+
 <h2 id="typographic-pseudos">
 Typographic Pseudo-elements</h2>