Notes in ispinlock

kern/ispinlock.c

@@ -18,6 +18,7 @@ void spin_unlock(spinlock_t *l) {
 }
 
 // Optional: tiny pause/backoff (works even if Zihintpause isn't present).
+// See: https://github.com/riscv/riscv-isa-manual/blob/main/src/zihintpause.adoc
 void cpu_relax(void) {
 #if defined(__riscv_zihintpause)
     __asm__ volatile("pause");