Add a note about how to check for bare < and & in SGML docs, before the

knowledge disappears again.

doc/src/sgml/Makefile

@@ -2,7 +2,7 @@
 #
 # PostgreSQL documentation makefile
 #
-# $PostgreSQL: pgsql/doc/src/sgml/Makefile,v 1.107 2008/08/25 15:34:05 alvherre Exp $
+# $PostgreSQL: pgsql/doc/src/sgml/Makefile,v 1.108 2008/10/04 02:19:08 tgl Exp $
 #
 #----------------------------------------------------------------------------
 
@@ -53,6 +53,9 @@ endif
 # Enable some extra warnings
 # -wfully-tagged needed to throw a warning on missing tags 
 # for older tool chains, 2007-08-31
+# Note: try "make SPFLAGS=-wxml" to catch a lot of other dubious constructs,
+# in particular < and & that haven't been made into entities.  It's far too
+# noisy to turn on by default, unfortunately.
 override SPFLAGS += -wall -wno-unused-param -wno-empty -wfully-tagged
 
 ##