message: Clarify Editable CFG semantics with comment
  Summary: It's not entirely clear to new users that the editable CFG empties out the IRCode object. Let's try to help there.
diff: * `code->build_cfg(true)`
  *
  * The editable CFG takes the MethodItemEntries from the IRCode object and moves
  - * them into the blocks. You can make all sorts of changes to the CFG and when
  + * them into the blocks. The editable CFG steals the code out of the IRCode
  + * object. After you've built the CFG in editable mode, you should use the CFG,
  + * not the IRCode. The IRCode is empty while the editable CFG exists.
  + *
  + * You can make all sorts of changes to the CFG and when
  * you're done, move it all back into an IRCode object with
  *
  * `code->clear_cfg()`
  