doc: kernel: mark k_event apis as being ISR safe

Most of the k_event APIs are ISR-safe, with the exception of

* k_event_init() - object should be initialized before ISR context
* k_event_wait(), k_event_wait_all() - only ISR safe with K_NO_WAIT

The last two functions already check for K_NO_WAIT when in ISR context
so this is very much just updating documentation to reflect the
current state.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>