Fix warning and improve description of ignore_parameter test

See https://github.com/swig/swig/pull/2205#issuecomment-1052648190

Examples/test-suite/ignore_parameter.i

@@ -11,6 +11,10 @@
 %typemap(argout) int bb "called_argout = 1;"
 
 %ignore unignorable;
+// Don't let SWIG expand this function with its default parameter: instead,
+// always "require" the version of the function with a single argument, but
+// then ignore it using `numinputs=0`.
+%ignore audi();
 
 %inline %{
 // constant for detecting correct "argout" call