clean up the change_objective! overloading, add doc

3 jobs for mk-lh-remove-duplicate in 6 minutes and 39 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed docker:julia1.7 #319853

00:05:06

passed format #319854
privileged

00:01:26

 
  Documentation
passed doc-tests-pr:julia1.7 #319855

00:01:33