cmd ref: review description of init and related commands (per --subdir)

[jorgeorpinel]

Extracted from #1022 (review)

• Update sync commands per "data sync commands without specific target: Stages for subdirs will not be gathered"
• May be we should consider moving some part to the User Guide.

Discussion part:

I also think (already discussed with Ivan) that the result here is a little too long. We went from 2 paragraphs to 21 plus block codes. It's not necessarily a problem because all of this info is valuable, but I think in part it can be summarized at least a little, and possibly extract these advanced cases into the user guide (as suggested above). We even state that they are "rare", so I'm not sure it's worth explaining them in detail in the description of this cmd ref. Another option is to make them into examples and keep the direct links from the description. Food for thought.

[jorgeorpinel]

We should also review the text of what the --subdir and --no-scm options do (motivation) as well as examples because they may be incorrect as explained in iterative/dvc#2901 (comment).