Document that non-IR units are passed by non-const reference instead of pointer in general

PiperOrigin-RevId: 266583029

Author: joker-eph

g3doc/DeveloperGuide.md

@@ -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