Better prompt to help user choose right parameters to improve dumpling's effiency

[lichunzhu]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, users don't know that --rows can enable inner-table concurrency.

Describe the feature you'd like:

1. Add hint in dumpling help info.
2. Add hint in dumpling's log when --rows is not set, like we did for tikv_gc_lifetime.
3. A more radical way is to set --rows for default, which is better after support both filesize and rows arguments #177 is merged. Need more discussion.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Optimization: