Add new terms rule type to Detections API create/update rule docs

[marshallmain]

Adds API docs for the new terms rule type, introduced in elastic/kibana#134526 (stack v8.4.0). Support for multiple values in new_terms_fields was added in 8.6 (elastic/kibana#143943), so if we backport as far as 8.4 and 8.5 we should modify the Must contain 1-3 field names. text to reflect the limitation (something like Must contain 1 field name. instead).

[github-actions]

Documentation previews:

• ✨ HTML diff
• 📙 Elastic Security Guide

[mergify]

This pull request does not have a backport label. Could you fix it @marshallmain? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[natasha-moore-elastic]

Thanks for raising the PR, @marshallmain! LGTM, just a couple of suggestions:

1. Can we add the following text to the list of rule types at the start of the rules-api-create page (on line 36, after Machine learning rules):

New terms: Generates an alert for each new term detected in source documents within a specified time range.

2. Can we add request and response examples for creating a new terms rule to the rules-api-create page? Just to keep the consistency, as the page contains examples for all other rule types.

[natasha-moore-elastic]

Adds API docs for the new terms rule type, introduced in elastic/kibana#134526 (stack v8.4.0). Support for multiple values in new_terms_fields was added in 8.6 (elastic/kibana#143943), so if we backport as far as 8.4 and 8.5 we should modify the Must contain 1-3 field names. text to reflect the limitation (something like Must contain 1 field name. instead).

Thanks for the heads-up – once we merge this PR, I can raise a separate one to update the description to Must contain 1 field name. and backport that update to 8.4 and 8.5 only.

[natasha-moore-elastic]

Thanks for raising the PR, @marshallmain! LGTM, just a couple of suggestions:

1. Can we add the following text to the list of rule types at the start of the rules-api-create page (on line 36, after Machine learning rules):

New terms: Generates an alert for each new term detected in source documents within a specified time range.

2. Can we add request and response examples for creating a new terms rule to the rules-api-create page? Just to keep the consistency, as the page contains examples for all other rule types.

Hey @marshallmain, just flagging these two points again in case you missed them.