message: doc: explain logical assignment option
diff: @@ -180,7 +180,10 @@ decaffeinate could be improved, feel free to file an issue on the [issues] page.
  - `--loose-js-modules`: Allow named exports when converting to JS modules.
  - `--disallow-invalid-constructors`: Give an error when constructors use `this`
  before `super` or omit the `super` call in a subclass.
  -- `--optional-chaining`: Do not transpile optional chaining
  +- `--optional-chaining`: Do not transpile optional chaining.
  +- `--logical-assignment`: Use the ES2021 [logical
  + assignment](https://github.com/tc39/proposal-logical-assignment) operators
  + `&&=`, `||=`, and `??=`.
  For more usage details, see the output of `decaffeinate --help`.
  