explain why we need to disable hwasan

Bug: 254365701
Change-Id: I2d67903608593fe3aeafea347a6c4a375a4295c2
diff --git a/sigchainlib/sigchain_test.cc b/sigchainlib/sigchain_test.cc
index 7d5d5f0..5e9c7fe 100644
--- a/sigchainlib/sigchain_test.cc
+++ b/sigchainlib/sigchain_test.cc
@@ -256,6 +256,8 @@
 }
 
 #if defined(__aarch64__)
+// The test intentionally dereferences (tagged) null to trigger SIGSEGV.
+// We need to disable HWASAN since it would catch the dereference first.
 DISABLE_HWASAN void fault_address_tag_impl() {
   struct sigaction action = {};
   action.sa_flags = SA_SIGINFO;