[doc, aon_timer] Clarify when wakeup and interrupt triggered
[doc, aon_timer] Clarify when wakeup and interrupt triggered

Explicitly state wakeup and watchdog counts must be cleared along with
the wakeup and interrupt events to avoid immediate retriggering.

Signed-off-by: Greg Chadwick <3e4c3d6d87c017d1a2fa161477653b4039c6b2fb@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan