docs: PG 13 relnotes: add links and SGML formatting

doc/src/sgml/README.links

@@ -12,10 +12,6 @@ Intra-document Linking
         is a refentry;  has no close tag
 	http://www.oasis-open.org/docbook/documentation/reference/html/xref.html
 
-<link>
-	use to supply text for the link, requires </link>
-	http://www.oasis-open.org/docbook/documentation/reference/html/link.html
-
 linkend=
 	controls the target of the link/xref, required
 
@@ -23,6 +19,10 @@ endterm=
 	for <xref>, allows the text of the link/xref to be taken from a
 	different link target title
 
+<link>
+	use to supply text for the link, only uses linkend, requires </link>
+	http://www.oasis-open.org/docbook/documentation/reference/html/link.html
+
 
 External Linking
 ----------------

doc/src/sgml/release.sgml

@@ -6,14 +6,14 @@ Typical markup:
 &<>                             use & escapes
 PostgreSQL                      <productname>
 postgresql.conf, pg_hba.conf    <filename>
-[A-Z][A-Z_ ]+[A-Z_]             <command>, <literal>, <envar>, <acronym>
-[A-Za-z_][A-Za-z0-9_]+()        <function>
+\<[A-Z][A-Z_ ]+[A-Z_]\>         <command>, <literal>, <envar>, <acronym>
+\<[A-Za-z_][A-Za-z0-9_]+()      <function>
 \-\-?[A-Za-z_]+[-A-Za-z_]*      <option> (use backslashes to avoid SGML markup)
-[A-Za-z_]+/[A-Za-z_]+           <filename>
+\<[A-Za-z_]+/[A-Za-z_]+\>       <filename>
 psql                            <application>
 pg_[A-Za-z0-9_]+                <application>, <structname>
-[A-Z][A-Z][A-Z_ ]*              <type>
-[a-z]+_[a-z_]+                  <varname>, <structfield>
+\<[A-Z][A-Z][A-Z_ ]*\>          <type>
+\<[a-z]+_[a-z_]+\>              <varname>, <structfield>
                                 <systemitem class="osname">
 
 non-ASCII characters            find using grep -P '[\x80-\xFF]'