improve documentation for 'atomic verify'

[miabbott]

Currently, the only documentation available (that I was able to find) for atomic verify is the inline help:

# atomic verify --help
usage: atomic verify [-h] [--no-validate] [--storage STORAGE] [-v] image

positional arguments:
  image              container image

optional arguments:
  -h, --help         show this help message and exit
  --no-validate      disable validating system images
  --storage STORAGE  Specify the storage of the image. If not specified and
                     there are images with the same name in different
                     storages, you will be prompted to specify.
  -v, --verbose      Report status of each layer

Which by itself is not entirely helpful, since it doesn't describe what it does.

Additionally, there is no man page for that sub-command.