Show syntax for specifying multiple shorthand pairs

This is a frequent enough question that it makes sense to
document that multiple list values are specified as
``--arg a=b,c=d a=b,c=d`` vs. ``--arg a=b,c=d``.

For example, the help now looks like:

```
       --tags (list)
          One or more tags. The value parameter is required, but if you  don't
          want  the  tag to have a value, specify the parameter with no value,
          and we set the value to an empty string.

       Shorthand Syntax:

            Key value pairs, with multiple values separated by a space.
          --tags Key=string,Value=string Key=string,Value=string
```

cc @danielgtaylor @kyleknap