Add comment for UnixLock

The Lock/Unlock mechanism on Unix is a manual, hand-coded implementation
of PTHREAD_MUTEX_RECURSIVE. We avoid using the PTHREAD_MUTEX_RECURSIVE
directly because it exhibits critical bugs, such as deadlocks on
certain older systems(Linux, Solaris, or macOS).
followup #2219

src/Mayaqua/Unix.c

@@ -1849,6 +1849,8 @@ void UnixUnlockEx(LOCK *lock, bool inner)
 }
 
 // Lock
+// Recursive locking is implemented manually instead of using PTHREAD_MUTEX_RECURSIVE.
+// See: https://github.com/SoftEtherVPN/SoftEtherVPN/pull/2219
 bool UnixLock(LOCK *lock)
 {
 	pthread_mutex_t *mutex;