Make it more clear that denied discretionary bonus needs feedback

Why is this change being made?

I recently nominated a team member for a bonus and was under the wrong impression that it was denied because of the team member leaving (https://gitlab.com/gitlab-com/people-group/employment/issues/1594#note_249822630). The corresponding executive did not give me feedback that or why the bonus was denied. This update to the handbook tries to make the responsibility more clear.