[ion] Add comment on default collectRegisters implementation

ion/src/shared/collect_registers.cpp

@@ -7,6 +7,10 @@ namespace Ion {
  * LTO inlining can make regs lower on the stack than some just-allocated
  * pointers. */
 __attribute__((noinline))uintptr_t collectRegisters(jmp_buf buf) {
+  /* TODO: we use setjmp to get the registers values to look for python heap
+   * root. However, the 'setjmp' does not guarantee that it gets all registers
+   * values. We should check our setjmp implementation for the device and
+   * ensure that it also works for other platforms. */
   setjmp(buf);
   int dummy;
   return (uintptr_t)&dummy;