doc PG 17 relnotes: add links to documentation sections

Also slightly improve markup instructions.  Indentation is still needed.

Backpatch-through: master

doc/src/sgml/release.sgml

@@ -16,7 +16,8 @@ pg_[A-Za-z0-9_]+                <application>, <structname>
 \<[a-z]+_[a-z_]+\>              <varname>, <structfield>
                                 <systemitem class="osname">
 
-non-ASCII characters            find using grep -P '[\x80-\xFF]'
+non-ASCII characters            find using grep -P '[\x80-\xFF]' or
+                                  (remove 'X') grep -X-color='auto' -P -n "[\x80-\xFF]"
                                 convert to HTML4 named entity (&) escapes
 
         official:      http://www.w3.org/TR/html4/sgml/entities.html
@@ -25,16 +26,13 @@ non-ASCII characters            find using grep -P '[\x80-\xFF]'
                        http://www.zipcon.net/~swhite/docs/computers/browsers/entities_page.html
                        https://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references
 
-        We cannot use UTF8 because back branches still use SGML Docbook,
-        which does not support it.  Also, rendering engines have to
+        We cannot use UTF8 because rendering engines have to
         support the referenced characters.
 
         Do not use numeric _UTF_ numeric character escapes (&#nnn;),
         we can only use Latin1.
 
         Example: Alvaro Herrera is &Aacute;lvaro Herrera
-        Find non-ASCII characters (remove 'X'):
-             grep -X-color='auto' -P -n "[\x80-\xFF]"
 
 wrap long lines