[c/en] clarify common tripping point of newbies

int foo () {
  printf("bar\n");
  int x; // this is not valid in C89+
}

c.html.markdown

@@ -75,6 +75,9 @@ int main (int argc, char** argv)
   // Types
   ///////////////////////////////////////
   
+  // All variables MUST be declared at the top of the current block scope
+  // we declare them dynamically along the code for the sake of the tutorial
+
   // ints are usually 4 bytes
   int x_int = 0;