doc: clarify the date standard output formats

doc: clarify the date standard output formats

* src/date.c (usage): Use FMT rather than TIMESPEC as the parameter,
since it's simpler to understand and can be better aligned.
Give an example for the --iso-8601 output format.
Adjust the example used for the 3 standard formats to be unambiguous
with respect to day/mon ordering and use of leading zeros in the time.
Reorder the options descriptions slightly, so that the
3 standards options are together.
Indent the multi-line descriptions so that grouping is obvious.
Remove a redundant description of the --rfc-3339 format,
which is obvious in the existing example.
Separate these 3 standards options to their own translatable string
to simplify translation.
Change 'date and time' to 'date/time' in the --iso-8601 description
to be consistent with --rfc-3339 and to help avoid the implication
that the time is always output or even output by default.
Fixes http://bugs.gnu.org/20203