message: Document that non-IR units are passed by non-const reference instead of pointer in general
diff: @@ -6,10 +6,13 @@ methods).
  ## Style guide
  -MLIR follows the [LLVM style](https://llvm.org/docs/CodingStandards.html) guide
  -except:
  +MLIR follows the [LLVM style](https://llvm.org/docs/CodingStandards.html) guide.
  +We also adhere to the following, that deviates from, or isn't specified in the
  +LLVM style guide:
  * Adopts [camelBack](https://llvm.org/docs/Proposals/VariableNames.html);
  +* Except for IR units (Region, Block, and Operation), non-nullable output
  + argument are passed by non-const reference in general.
  ## Pass name and other command line options
  