Use a command class's __doc__ if overview does not exist

[lengau]

What needs to get done

The overview attribute is often duplicative of a command class's docstring. If the overview attribute doesn't exist, use the command's docstring instead.

Why it needs to get done

Helps make automatic documentation better, since it'll display the command docs.

[lengau]

The first line of __doc__ could also become help_msg if it's missing.