[selectors-4] Restore text about :lang(*) matching that was accidentally removed by #4212. Fixes #6915.

Author: tabatkins

selectors-4/Overview.bs

@@ -2053,6 +2053,9 @@ The Language Pseudo-class: '':lang()''</h3>
 	The <a>language range</a> does not need to be a valid language code to
 	perform this comparison.
 
+	For this purpose, a wildcard [=language range=] (<code>"*"</code>)
+	does not match elements whose language is not tagged (e.g. <code>lang=""</code>),
+	but does match elements whose language is tagged as undetermined (<code>lang=und</code>).
 	A [=language range=] consisting of an empty string
 	('':lang("")'')
 	matches (only) elements whose language is not tagged.