[MLIR] Improve tutorial to make it clear that walk() is visiting the root op (NFC)

Author: joker-eph

mlir/docs/Tutorials/UnderstandingTheIRStructure.md

@@ -178,7 +178,7 @@ inside a single block (or a single region), however it is frequently interesting
 to traverse the IR in a nested fashion. To this end MLIR exposes the `walk()`
 helper on `Operation`, `Block`, and `Region`. This helper takes a single
 argument: a callback method that will be invoked for every operation recursively
-nested under the provided entity.
+nested under the provided entity (as well as this initial operation).
 
 ```c++
   // Recursively traverse all the regions and blocks nested inside the function