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