man/sd_event_{add_io,add_time,add_signal,now}: various small fixes - remove things which are clear from the context - 0 is a valid descriptor number, hence "positive" → "non-negative" - "positive" means greater than zero, hence "positive non-zero" → "positive" - use oxford comma - reword some things for clarity

sd-event: check clock argument to sd_event_now() sd_event_now() is a public function, so we must check all arguments for validity. Update man page and add tests. Sample debug message: Assertion 'IN_SET(clock, CLOCK_REALTIME, CLOCK_REALTIME_ALARM, CLOCK_MONOTONIC, CLOCK_BOOTTIME, CLOCK_BOOTTIME_ALARM)' failed at src/libsystemd/sd-event/sd-event.c:2719, function sd_event_now(). Ignoring.

man: fix typos

man: fully document sd-event interfaces This completes the set of man pages for sd-event and contains some minor other fixes for other man pages too. The sd_event_set_name(3) man page is renamed to sd_event_source_set_description(3), which is the correct name of the concept today.