Add -wnet to SP invocations

This causes a warning when accidentally backpatching an XML-style
empty-element tag like <xref linkend="abc"/>.

doc/src/sgml/Makefile

@@ -72,10 +72,11 @@ endif
 # Enable some extra warnings
 # -wfully-tagged needed to throw a warning on missing tags
 # for older tool chains, 2007-08-31
+# -wnet catches XML-style empty-element tags like <xref linkend="abc"/>.
 # 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
+override SPFLAGS += -wall -wno-unused-param -wno-empty -wfully-tagged -wnet
 
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