repro: improvements to targets arg and -R option

[jorgeorpinel]

Extracted from #1572 (comment)
And per private discussion with Ruslan rel. to iterative/dvc/issues/4292

• Update targets description to Stage or path to dvc.yaml or .dvc file to reproduce for now,
  see resolution of repro: enforce targets (and update desc) when -R is present? [qa] dvc#4292
• This should be clarified in the -R option desc: It expands what the targets argument accepts (directory paths).
  Make sure options -c and -P are also clear and complete, as they also allow to target directories in different ways.
• No example in the doc for using repro with a specific target stage or .dvc file is provided... I think this should be done