repro: improve targets help output as it relates to dvc.yaml files?

[jorgeorpinel]

Extracted from and related to #4292

The targets param accepted by dvc repro is described (in its help output) as Stages to reproduce. 'dvc.yaml' by default. This message is confusing as dvc.yaml is not a stage. In docs we're using Stage or .dvc file to reproduce but that's not correct either because apparently paths to dvc.yaml files are also accepted.

• Should we use Stage or path to dvc.yaml or .dvc file to reproduce instead?
• ^ still doesn't cover the case of accepting directory paths with -R, but that we can explain in that argument's description:

-  -R, --recursive       Reproduce all stages in the specified directory.
+  -R, --recursive       Reproduce all stages in the specified directories.
+                        These can be given as `targets` arguments.

and I'll explain this special case further in the cmd ref. Options.

OPTIONALLY, should repro NOT accept dvc.yaml files as targets, and change the default description to "all stages in dvc.yaml"?

p.s. which other commands accept dvc.yaml file paths as targets?

[jorgeorpinel]

p.s. which other commands accept dvc.yaml file paths as targets?

Sync commands (push, fetch, pull) seem to accept them... Should probably review their help output and docs for consistency too. Any others? All of them (the ones that accept targets) perhaps?