[doxygen] Clarify doxygen.conf PREDEFINED variable.

doc/doxygen.conf

@@ -1663,15 +1663,12 @@ INCLUDE_FILE_PATTERNS  =
 # ns-3:
 #
 # We predefine NS3_ASSERT_ENABLE and NS3_LOG_ENABLE so doxygen sees
-# the working definitions.
+# the working definitions.  (These are normally defined by waf
+# in ns3/core-config.h)
 #
 # Function like macros at file global scope typically need to be here,
 # since doxygen confuses invocations of these macros for function
 # definitions.
-#
-# We also list the generic logging macros to suppress the "Referenced by"
-# sections.  Since these macros are so heavily used the references are
-# pages long.
 
 PREDEFINED             = \
                          NS3_ASSERT_ENABLE \