Even when you document very frugal, have limited time or resources for documentation available… you should follow one of the most essential rules of architecture documentation:
Always describe level-1 of the building block view
makes the top-level structure of building blocks explicit!
very often remains quite stable over time - as the top-level system decomposition often has little or no volatility.
therefore causes only little maintenance or change effort…
needs to be consistent (with respect to external interfaces) to the system scope and context (see tip 5-4 (consistency of level-1 to external interfaces))