Clarify why we ignore EINVAL when flushing a file descriptor

src/io/SDL_iostream.c

@@ -502,6 +502,7 @@ static bool SDLCALL fd_flush(void *userdata, SDL_IOStatus *status)
         result = SDL_fdatasync(iodata->fd);
     } while (result < 0 && errno == EINTR);
 
+    // We get EINVAL when flushing a pipe, just make that a no-op
     if (result < 0 && errno != EINVAL) {
         return SDL_SetError("Error flushing datastream: %s", strerror(errno));
     }