Update documentation when we make --delete-tostring-package-uri defaulted for certain modes #52759
Description
flutter/engine#17068 will add a new option to the frontend compiler to let us remove toString options, along with an annotation to mark a method as keep even if we applied the flag.
We should update the docs on the annotation if and when we make this option on by default for, e.g. release or profile modes, to reflect when it's on.