pull: target message

[dmpetrov]

I heard about this issue from 2 independent users. They didn't know that dvc support pull for a file.

-R option is minor and looks a bit confusing. Let's remove it from the target description.

• Update docs if needed
  cmd: document target granularity for push/pull/etc, et al. dvc.org#1384

[codecov]

Codecov Report

Merging #4192 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4192   +/-   ##
=======================================
  Coverage   92.60%   92.60%           
=======================================
  Files         162      162           
  Lines       11312    11312           
=======================================
  Hits        10476    10476           
  Misses        836      836           

Impacted Files	Coverage Δ
dvc/command/checkout.py	93.02% <ø> (ø)
dvc/command/data_sync.py	100.00% <ø> (ø)

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[efiop]

Need to update it in-sync with https://github.com/iterative/dvc.org/pull/1384/files . Including fetch/push/status. We'll take it from here.