Issue #121: Adding info to `site backup` docstring
Issue #121: Adding info to `site backup` docstring

The `list` function was not listed as part of the help information
for the backup command.

I also re-wrote a couple of the docstring lines to make the format
more consistent with other docstrings throughout Terminus.

Signed-off-by: Elliot Voris <dee6300e151043f915cc24dbc1409935bc4ae592@voris.me>

reynoldsalec/cli,labboy0276/cli,mkudenko/cli,Blueprint-Marketing/cli,dustinleblanc/terminus,labboy0276/cli,pantheon-systems/cli,hugovk/cli,devekko/cli,hugovk/cli,pantheon-systems/cli,dustinleblanc/terminus,hugovk/cli,devekko/cli,rvtraveller/terminus,Blueprint-Marketing/cli,pantheon-systems/cli,Blueprint-Marketing/cli,rvtraveller/terminus,reynoldsalec/cli,dustinleblanc/terminus,rvtraveller/terminus,mkudenko/cli,reynoldsalec/cli,mkudenko/cli,devekko/cli,labboy0276/cli