[Request]: Add list of valid action variables to APM alerting topics

[dedemorton]

Description

Most of the topics about creating alerting rules for observability provide a list of action variables that are valid in that rule. The APM rules documented (documented under APM in the Kibana Guide) should, but do not. We are documenting valid action variables for each rule because this was a specific request by users who wanted to to see this information in the docs.

This is related to the work done in #3693).

NOTE: I started to update the content in place (where it lives in the Kibana Guide), but that won't work because the list of valid action variables is not the same across all the APM-related rules. I think it would be better to wait until after the rules are restructured before trying to make these changes.

Resources

n/a

Which documentation set does this change impact?

Stateful and Serverless

Feature differences

n/a

What release is this request related to?

N/A

Collaboration model

The documentation team

Point of contact.

Main contact: TBD

Stakeholders: