date.1: Fix some style issues and examples

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

(cherry picked from commit 00133d5c5e6aae01663eca40993abd5e6137425b)