Add comment about assumed pthread API

Author: Patrick-6

tests/genmc/fail/shims/mutex_double_unlock.rs

@@ -2,6 +2,8 @@
 //@error-in-other-file: Undefined Behavior
 
 // Test that GenMC can detect a double unlock of a mutex.
+// This test will cause an error even if the program actually would work entirely fine despite the double-unlock
+// because GenMC always assumes a `pthread`-like API.
 
 #![no_main]