[DOCS] Add examples in watchers definitions that uses throttle_period_in_millis

[laraMorenoIgle]

Description of the problem including expected versus actual behavior:

When defining an action in a watcher you can configure a throttle period.

In the documentation all the examples use throttle_period parameter, but you could use throttle_period_in_millis instead, since, as per the code both parameters are accepted.
However, throttle_period_in_millis parameter is mentioned neither in the examples related to watchers in the Elasticsearch documentation nor anywhere in the docs.

Examples in the doc using throttle_period
https://www.elastic.co/guide/en/elastic-stack-overview/7.2/actions-slack.html
https://www.elastic.co/guide/en/elastic-stack-overview/7.1/actions.html#actions-ack-throttle

Example in the discussion forum using throttle_period_in_millis
https://discuss.elastic.co/t/how-to-set-a-time-range-in-the-watcher/93679/4

It would be good to include examples in the documentation using throttle_period_in_millis too.

[elasticmachine]

Pinging @elastic/es-docs

[elasticmachine]

Pinging @elastic/es-core-features

[jrodewig]

Thanks for opening this issue @laraMorenoIgle.

I opened #45607 to document the throttle_period_in_millis parameter. However, I'm hesitant to update any of the existing examples.

It seems that throttle_period is likely a better default for most users. But I'm not that experienced with Watcher and its use cases. If you feel strongly that throttle_period_in_millis should be included in an example, I can look at maybe adding a new one.