Document test is for libc++ hardening

Change-Id: I8cc3c8a622d9807ca477184683b29900ca420267
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4889668
Auto-Submit: danakj <danakj@chromium.org>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1201685}

base/std_clamp_unittest.cc

@@ -21,6 +21,7 @@ struct AnotherType {
   int some_other_int;
 };
 
+// Verify libc++ hardening terminates instead of UB with invalid clamp args.
 TEST(ClampTest, Death) {
   EXPECT_DEATH_IF_SUPPORTED(std::ignore = std::clamp(3, 10, 0), "");
   EXPECT_DEATH_IF_SUPPORTED(std::ignore = std::clamp(3.0, 10.0, 0.0), "");