a more detailed explanation for some of the litescope_cli arguments

[gsomlo]

Most of the arguments to litescope_cli have pretty self-explanatory "help" text. However, it might be worth providing more substantial details (e.g., a whole paragraph, in either a separate document, or via some other more python-esque/inline form) for the following:

• --group
• --subsampling
• --offset
• --length (also, is this related -- and how -- to the depth argument to the SoC-side LiteScopeAnalyzer?