doc: trim "mostly repetitive" docs

This trims of the docs of routines like 'checked_sub', 'saturating_sub'
and 'since'. Instead, we just say, very briefly, how they differ from
'checked_add', 'saturating_add' and 'until', respectively.

This commit also has some other documentation clean-ups, like replacing
most uses of `jiff::civil::Date::constant` with `jiff::civil::date`.