Add comment to encodingOverhead

src/objective-c/tests/InteropTests.m

@@ -90,6 +90,9 @@
   return nil;
 }
 
+// This number indicates how many bytes of overhead does Protocol Buffers encoding add onto the
+// message. The number varies as different message.proto is used on different servers. The actual
+// number for each interop server is overridden in corresponding derived test classes.
 - (int32_t)encodingOverhead {
   return 0;
 }