usbconfig.8: Improve style and fix examples

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

(cherry picked from commit 5eafaf9e68894cda4e8dca5f363be0b4f15b5cd6)