Documentation: Update Polling API

Changes:
1. the number of events being checked did not equal those defined
in the `events` array.
2. Use `k_poll_signal_check()` instead of accessing fields directly.
3. Use `k_poll_signal_reset()` to reset the signal instead of
accessing fields directly.

I'm assuming the examples predate `k_poll_signal_check()` and
`k_poll_signal_reset()`

Signed-off-by: Glenn Andrews <glenn.andrews.42@gmail.com>