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

usr.sbin/periodic/periodic.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 20, 2016
+.Dd June 18, 2020
 .Dt PERIODIC 8
 .Os
 .Sh NAME
@@ -32,7 +32,10 @@
 .Nd run periodic system functions
 .Sh SYNOPSIS
 .Nm
-.Ar directory ...
+.Sm off
+.Cm daily | weekly | monthly | security | Ar directory
+.Sm on
+.Ar ...
 .Sh DESCRIPTION
 The
 .Nm
@@ -42,22 +45,22 @@ to execute shell scripts
 located in the specified directory.
 .Pp
 One or more of the following arguments must be specified:
-.Bl -tag -width ".Pa monthly"
-.It Pa daily
+.Bl -tag -width "directory"
+.It Cm daily
 Perform the standard daily periodic executable run.
 This usually occurs early in the morning (local time).
-.It Pa weekly
+.It Cm weekly
 Perform the standard weekly periodic executable run.
 This usually occurs very early on Saturday mornings.
-.It Pa monthly
+.It Cm monthly
 Perform the standard monthly periodic executable run.
 This usually occurs on the first day of the month.
-.It Pa security
+.It Cm security
 Perform the standard daily security checks.
 This is usually spawned by the
-.Pa daily
+.Cm daily
 run.
-.It Ar path
+.It Ar directory
 An arbitrary directory containing a set of executables to be run.
 .El
 .Pp