event.h: Documented some newly added portions better

After the merge of v2023.10-rc4 to next include/event.h needs to be
fully documented in order for documentation builds to complete.  Rewords
two of the event_t descriptions to be docbook style and better match the
rest of this enum.  Fix two typos (flag->flags) in other comments.

Signed-off-by: Tom Rini <trini@konsulko.com>