lock: mention that dvc update is now also setting locked: True

[efiop]

iterative/dvc#2238

[jorgeorpinel]

More details (per #464 (review)) update dvc import-get cmd ref around phrase "A pipeline can be triggered to re-execute based on..." Explanation:

"...by default all external dependencies import stages (from external data sources) are going to be locked (lock: true and -rev-lock: <git-commit-hash> in the DVC-file). It means when you run dvc repro or dvc status they are not going to check the changes automatically. You will need to explicitly call dvc update to fetch the new data, update the -rev-lock field."

[jorgeorpinel]

Another note here, from Discord:

dvc unlock "won't cause problems. If you dvc unlock imported stage then it will simply be updated by dvc repro next time. and it will check dependency on dvc status and stuff.

for a locked stage, dvc update does dvc unlock + dvc repro + dvc lock basically".

[jorgeorpinel]

Note that jorgeorpinel/dvc.org@57598c3...a06714a should close this issue, as part of PR #490.