docs comments: clarify why not to use UTF8 still in docs

Back branches still are SGML.

doc/src/sgml/release.sgml

@@ -26,10 +26,11 @@ non-ASCII characters            find using grep -P '[\x80-\xFF]'
                        http://www.zipcon.net/~swhite/docs/computers/browsers/entities_page.html
                        http://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references
 
-        we cannot use UTF8 because SGML Docbook does not support it
+        We cannot use UTF8 because back branches still use SGML Docbook,
+	which does not support it.
 
-        do not use numeric _UTF_ numeric character escapes (&#nnn;),
-        we can only use Latin1
+        Do not use numeric _UTF_ numeric character escapes (&#nnn;),
+        we can only use Latin1.
 
         Example: Alvaro Herrera is Álvaro Herrera