Add notes for discarding private-visible functions in the Toy tutorial chapter 4.
Hi, I'm a first-time contributor without the commit accesses. This commit was inspired while I'm following the tutorial to make the Toy language. After I finished the chapter 4, I found my compiler could not discard the function body of func @multiply_transpose, so that the shape inference pass would fail in this function. I'm a beginner for LLVM, it really took me a long time to read the MLIR code and find the visibility problem of my Toy compiler. So I'm wondering whether we could put this note into the docs for friendliness. Thanks!