feat: document placeholder attribute on templates properties

[barmac]

Description

This PR documents the placeholder attribute added to the element templates via bpmn-io/bpmn-js-element-templates#92

The changes will be available in the upcoming Desktop Modeler 5.24 release, and also in Web Modeler SaaS as soon as released.

Closes camunda/camunda-modeler#4331

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.