Improve periodic(8) manual page presentation

Improve periodic(8) manual page presentation

- Update synopsis to present all available arguments.
- Consistently call the argument specifying an arbitrary directory a
  "directory".
- Do not put macros into -width argument to Bl. They do not expand there.
- Stylize command modifiers like "daily" with Cm instead of Pa. While
  technically periodic(8) operates on directories with such names, it is
  confusing from the perspective of the manual page reader as Pa and Ar are
  stylized the same way. Also, I cannot recall a single manual page where
  Pa would be used to describe the syntax of command-line arguments.

MFC after:	2 weeks