doc: Clarify k_poll() behavior with respect to signal synchronization

Bug #40189 tripped over an interesting synchronization scenario that
wasn't called out in the docs.  Poll signals are level triggered, and
if you're adjusting the level from a racing context (e.g. resetting it
before the next "event" from an ISR or another thread) the polling
thread might wake up but then miss the event.  Mention this case
explicitly in documentation.

Fixes #40189

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>