feat(components/modeler): document default-httl flag

[nikku]

Description

Documents the newly added default-httl flag, contributed via camunda/camunda-modeler#4147 and camunda/camunda-modeler#4162.

Depends on #3387

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply hold label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant /versioned_docs directory, or they are not for an already released version.
• I have added changes to the main /docs directory (aka /next/), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[github-actions]

👋 🤖 🤔 Hello! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.4/.

• docs/components/modeler/desktop-modeler/flags/flags.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.

The base branch was changed.

[nikku]

@barmac Please re-review. I had to update this PR and your review got dismissed 😭