api: improve existing docstrings per https://github.com/iterative/dvc/issues/3092#issuecomment-572394156
api: improve existing docstrings
per https://github.com/iterative/dvc/issues/3092#issuecomment-572394156

dmpetrov/dataversioncontrol,dmpetrov/dataversioncontrol,efiop/dvc,efiop/dvc