src/doc: improve man page and --help

- add missing environment variables to --help
- add missing flags to man page
- sort environment variables alphabetically
- add some highlighting to the man page
- remove stops from descriptions in --help for consistency
- few other minor tweaks to --help

PR-URL: https://github.com/nodejs/node/pull/10157


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>