docs: mention that syz-verifier is broken

Don't confuse the users by making it look like the tool works.

Fixes #5950.

Author: a-nogikh

docs/syz_verifier.md

@@ -1,3 +1,6 @@
+**`syz-verifier` is currently broken and cannot be compiled/used, see
+[the backlog issue](https://github.com/google/syzkaller/issues/5976).**
+
 # syz-verifier
 
 Many bugs are easy to detect: they might cause assertions failures, crash our