Update sig_stfl.h document for #1650 (#1655)

* update the stateful siganture header documentation

* catch the case when mutex is not set

* stress that only the Signing operation need to be locked/unlocked.

* make lock and unlock function to internal APIs.

src/sig_stfl/sig_stfl.c

@@ -1370,24 +1370,35 @@ OQS_API void OQS_SIG_STFL_SECRET_KEY_SET_mutex(OQS_SIG_STFL_SECRET_KEY *sk, void
 }
 
 /* OQS_SIG_STFL_SECRET_KEY_lock  */
-OQS_API OQS_STATUS OQS_SIG_STFL_SECRET_KEY_lock(OQS_SIG_STFL_SECRET_KEY *sk) {
+OQS_STATUS OQS_SIG_STFL_SECRET_KEY_lock(OQS_SIG_STFL_SECRET_KEY *sk) {
 	if (sk == NULL) {
 		return OQS_ERROR;
 	}
 	if (sk->lock_key == NULL) {
 		return OQS_SUCCESS;
 	}
 
+	// Try to lock the private key but the mutex is unset.
+	if (sk->mutex == NULL) {
+		return OQS_ERROR;
+	}
+
 	return (sk->lock_key(sk->mutex));
 }
 
 /* OQS_SIG_STFL_SECRET_KEY_unlock */
-OQS_API OQS_STATUS OQS_SIG_STFL_SECRET_KEY_unlock(OQS_SIG_STFL_SECRET_KEY *sk) {
+OQS_STATUS OQS_SIG_STFL_SECRET_KEY_unlock(OQS_SIG_STFL_SECRET_KEY *sk) {
 	if (sk == NULL) {
 		return OQS_ERROR;
 	}
 	if (sk->unlock_key == NULL) {
 		return OQS_SUCCESS;
 	}
+
+	// Try to unlock the private key but the mutex is unset.
+	if (sk->mutex == NULL) {
+		return OQS_ERROR;
+	}
+
 	return (sk->unlock_key(sk->mutex));
 }