check-ignore: update help output (#4358)
check-ignore: update help output (#4358)

* check_ignore: update help output
per https://github.com/iterative/dvc/pull/4282#pullrequestreview-460270636
and https://github.com/iterative/dvc/pull/4282#pullrequestreview-460272287

* check-ignore: more output string updates
per #4323 et al.

* check-ignore: min text update
to match iterative/dvc.org/pull/1673

Co-authored-by: Ruslan Kuprieiev <f6538312ce47017d734b1357aabc970ebf896e32@gmail.com>
efiop/dvc,dmpetrov/dataversioncontrol,dmpetrov/dataversioncontrol,efiop/dvc