Clarify that loplugin:unnecessaryparen doesn't want to warn about sizeof

...for whatever reason

Change-Id: Iaae919dc6c636c4846c548914e593cb8bd94fe8b

compilerplugins/clang/test/unnecessaryparen.cxx

@@ -61,6 +61,9 @@ int main()
     if ((s2 = s1)) {
         return 0;
     }
+
+    (void) sizeof (int);
+    (void) sizeof (x); // expect no warning (for whatever reason; for symmetry with above case?)
 };
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */