docs: improve xref description for xreflabel and refentry links

Author: bmomjian

doc/src/sgml/README.links

@@ -7,8 +7,9 @@ Intra-document Linking
 ----------------------
 
 <xref>
-	use to get chapter/section # from the title of the target
-	link, or xreflabel or refentry if defined at the target; has no close tag
+	use to get chapter/section number from the title of the target
+	link, or xreflabel if defined at the target, or refentrytitle if target
+        is a refentry;  has no close tag
 	http://www.oasis-open.org/docbook/documentation/reference/html/xref.html
 
 <link>