get/import: updates output strings, et al. (#3308)

* get/import: update cmd and url arg descs with correct behavior

* get/import: update path arg desc as well (See previous commit)

* get/imp: udpate help output with simpler terms
per #3277 (review)

* api: url->URL in a docstring

* metrics show: update help output per
per iterative/dvc.org#933 (review)

* diff: update `diff` and `metrics diff` help desc to match docs
per iterative/dvc.org#933 (review)

* diff: update autocomplete scripts
per #3308 (comment)

* diff: update help output again
per iterative/dvc.org#933 (review)

* diff/metrics diff: update help output yet again
per iterative/dvc.org#933 (comment)

Co-authored-by: Ruslan Kuprieiev <kupruser@gmail.com>