[NFC][clang] Fix header comment in llvm/test/MC/AArch64/local-bounds-single-trap.ll (#117642)

llvm/test/MC/AArch64/local-bounds-single-trap.ll was introduced in
#65972 to demonstrate that
nomerge did not work properly, which is documented in the header
comment.

#101549 fixed nomerge for trap
builtins and
ae6dc64
updated the test assertions, but not the header comment. This patch
updates the header comment accordingly.

Author: thurstond

llvm/test/MC/AArch64/local-bounds-single-trap.ll

@@ -1,7 +1,8 @@
 ; RUN: llc -O3 -mtriple arm64-linux -filetype asm -o - %s | FileCheck %s -check-prefix CHECK-ASM
-; What this test does is check that even with nomerge, the functions still get merged in
-; compiled code as the ubsantrap call gets lowered to a single instruction: brk.
-
+; This test checks that nomerge correctly prevents the traps from being merged
+; in the compiled code.
+; Prior to ae6dc64ec670891cb15049277e43133d4df7fb4b, this test showed that
+; nomerge did not work correctly.
 
 @B = dso_local global [10 x i8] zeroinitializer, align 1
 @B2 = dso_local global [10 x i8] zeroinitializer, align 1