[DOCS][Response Ops] Update create/update rule docs to reflect latest notify_when options

[ymao1]

As of this PR #144130 in 8.7, the top level notify_when field is now optional during rule create/update, not required as indicated in the docs: https://www.elastic.co/guide/en/kibana/current/create-rule-api.html, https://www.elastic.co/guide/en/kibana/current/update-rule-api.html. In addition, that PR added an action level frequency field with summary and notify_when entries that should be populated on rule create/update. We need to update the docs to reflect these updates.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[lcawl]

In general we're not updating those manually-maintained API pages anymore, which is why there's a link to the open API specification as the source of truth. As soon as we start publishing directly from the specifications, we'll be retiring these old pages. I'll double-check that the OAS is accurate.

[lcawl]

The notify_when property is accurately shown as required only within the context of the actions object in https://github.com/elastic/kibana/blob/main/x-pack/plugins/alerting/docs/openapi/bundled.json#L6741 and https://github.com/elastic/kibana/blob/main/x-pack/plugins/alerting/docs/openapi/bundled.json#L2691

[cnasikas]

I am closing this issue as the docs contain the latest API updates.