When submitting #135854 I've assumed that `RuntimeError` is
connected particularly to `{'CANCELLED'}` return status. Turned
out error is raised regardless of what return status is and it's
only based on the presence of error reports during operator
execution. Submitting a clarification for this.
Pull Request: https://projects.blender.org/blender/blender/pulls/138558
The effect of CANCELLED on the undo stack is quite obscure, and
mistakenly using it after doing some changes causes confusing
behavior. It's better to describe it explicitly in the docs.
