Clean up attributes in reference documentation

This is a bit unfortunate since they are quite useful as documentation, but
unfortunately Doxygen (as usual) doesn't manage to format things nicely, and
the noise makes the summaries unreadable.

doc/reference.doxygen.in

@@ -2044,7 +2044,11 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = SERD_API
+PREDEFINED             = SERD_API= \
+                         SERD_CONST_API= \
+                         SERD_NONNULL= \
+                         SERD_NULLABLE= \
+                         SERD_PURE_API
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The