Add more useful usage instructions for all subcommands (#4710)
Add more useful usage instructions for all subcommands (#4710)

* Add more useful usage instructions for all subcommands

Fixes #3875.

* Update usage instructions

Address PR comments.  Fixes #3875.

* Fix line length

* Suffix usage lines with two empty lines

Per review, this brackets all usage text in 2 newlines to match existing
text.

lmcro/letsencrypt,jsha/letsencrypt,letsencrypt/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,stweil/letsencrypt,jsha/letsencrypt