[ASan] add comment to blacklist file to test behavior introduced in r166283

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@166284 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/asan/tests/asan_test.ignore b/lib/asan/tests/asan_test.ignore
index 7bafa83..ea5c260 100644
--- a/lib/asan/tests/asan_test.ignore
+++ b/lib/asan/tests/asan_test.ignore
@@ -1,2 +1,3 @@
+# blacklisted functions for instrumented ASan unit test
 fun:*IgnoreTest*
 fun:*SomeOtherFunc*