Add help message for magisk --preinit-device

native/src/core/magisk.cpp

@@ -38,6 +38,7 @@ Advanced Options (Internal APIs):
    --sqlite SQL              exec SQL commands to Magisk database
    --path                    print Magisk tmpfs mount path
    --denylist ARGS           denylist config CLI
+   --preinit-device          resolve a device to store preinit files
 
 Available applets:
 )EOF");