[dv/alert_handler] update alert_handler doc
[dv/alert_handler] update alert_handler doc

The original alert_handler dv doc has a few WIP items. This PR updates
the sections in scb, coverage, and sequence.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@google.com>

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