Support note above/bellow of ClassName

[evandrocoan]

Currently I can use left and right:

  note left of MainController
    This is a model controller
  end note

But sometimes there is a lot of space above or bellow the diagram. Other times, it can be on the upper left, upper right or down left and down right corners. For example:

  note upper left of MainController
    This is a model controller
  end note

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Beyond that, PlantUml can try to guess where there is the most available space with:

  note auto of MainController
    This is a model controller
  end note