get/import: help output should reflect non-DVC Git repo support

[jorgeorpinel]

After #2889 and #3020, these comments can also obtain files that are tracked by Git but not DVC, however their help output looks:

$ dvc get -h
usage: dvc get [-h] [-q | -v] [-o [OUT]] [--rev [REV]] url path
...
positional arguments:
  url                   URL of Git repository with DVC project to download
                        from.
  path                  Path to a file or directory within a DVC repository.
$ dvc import -h
usage: dvc import [-h] [-q | -v] [-o [OUT]] [--rev [REV]] url path
...
positional arguments:
  url                   URL of Git repository with DVC project to download from.
  path                  Path to data within DVC repository.

$  dvc version
DVC version: 0.80.0
Python version: 3.7.6
Platform: Darwin-19.0.0-x86_64-i386-64bit
Binary: False
Package: brew

I think all url and path descriptions need to be updated.

[jorgeorpinel]

• And please update the cmd ref in the docs, of course 🙂
  UPDATE: Should be done in DOC: reflect the fact that get allows for non-DVC repositories dvc.org#912