dvc: standard way to refer to DVC-files

[jorgeorpinel]

"DVC file(s)" an similar -> "DVC-file(s)"

E.g. in dvc remove -h and dvc repro -h output~~. Anywhere else?~~ and in many more places!

Companion to iterative/dvc.org#321

• Have you followed the guidelines in our
  Contributing document?

• Does your PR affect documented changes or does it add new functionality
  that should be documented? If yes, have you created a PR for
  dvc.org documenting it or at
  least opened an issue for it? If so, please add a link to it.

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[pared]

If we want to refer to them as DVC-files, I approve. The question is shouldn't it be "stage files"?

[jorgeorpinel]

It's a good question about the term "stage files". I think both are valuable. I suggest keep both in the docs but since "DVC-file" is much more prevalent, I can replace all the "stage file" occurrences in this repo.

Will ask on discord to see what does the team think.