usbconfig.8: Improve style and fix examples

• Use Ar macros for arguments
• Stylize the argument synopsis to the -d flag
• Change the width of the list to one of the actual tags in the list
• Stylize "ugen" and "/dev/ugen" with Cm as those are constant strings, which are usually treated as command modifiers.
• Break long lines to reduce the number of warnings from linters
• Fix examples; the -d flag is now required when specifying the unit and the address with the "dot notation".

MFC after: 2 weeks