[Internal]: Document Synthetics Journey Timeout hard limit

[louisong]

Description

What: Synthetics Journey Timeout has a hard limit of 15mins.
Why: We should document this default limit for user reference/guidance.

Resources

For now, we documented this in a knowledge based article: https://support.elastic.dev/knowledge/view/10c028e4

Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

Feature differences

NA

What release is this request related to?

N/A

Serverless release

NA

Collaboration model

The documentation team

Point of contact.

Main contact: @shahzad31

Stakeholders: @drewpost