doc: Update README.links

The guideline to "not use text with <ulink> so the URL appears in
printed output" is obsolete, since the URL appears as a footnote in
printed output in that case.
Reported-by: Chapman Flack <chap@anastigmatix.net>
Discussion: https://www.postgresql.org/message-id/5C4B1F2F.2000106@anastigmatix.net

doc/src/sgml/README.links

 <!-- doc/src/sgml/README.links -->
 
-Linking within SGML documents can be confusing, so here is a summary:
+Linking within DocBook documents can be confusing, so here is a summary:
 
 
 Intra-document Linking
@@ -41,6 +41,5 @@ Guidelines
 ----------
 
 o  If you want to supply text, use <link>, else <xref>
-o  Do not use text with <ulink> so the URL appears in printed output
 o  Specific nouns like GUC variables, SQL commands, and contrib modules
    usually have xreflabels