date.1: Fix some style issues and examples

• Use Cm instead of Ar or Sq for command modifiers of the -v flag.
• Remove unnecessary "Ar ..." from the synopsis. It's not clear what it was referring to.
• Add missing arguments to the -f and -v flags.
• Stylize the dot before "ss" with Cm in the default format in the -f flag description.
• Set LC_ALL=C in the last example so that the output format of date(1) always matches the specified format of the -f flag not matter the locale.
• List the -f flag as optional in all usage lines in the synopsis.

MFC after: 3 days