doc: document problems with using xreflabel in XML docs

Reported-by: Peter Eisentraut

Discussion: https://postgr.es/m/8315c0ca-7758-8823-fcb6-f37f9413e6b6@2ndquadrant.com

Backpatch-through: master

Author: bmomjian

doc/src/sgml/README.links

@@ -47,3 +47,7 @@ Guidelines
   usually have xreflabels.
 
 - For an external link, use <ulink>, with or without link text.
+
+- xreflabels added to tags prevent the chapter/section for id's from being
+  referenced;  only the xreflabel is accessible.  Therefore, use xreflabels
+  only when linking is common, and chapter/section information is unneeded.