Draft: Docs: Add MSC4143 feature to Synapse config

[her001]

Content

It appears that some clients now fail when MatrixRTC transports endpoint isn't available. In Synapse, this requires enabling the msc4143_enabled experimental feature. This change adds the necessary config to the self hosting documentation.

Motivation and context

After figuring this out on my own, I discovered that there was an issue that describes the same behavior: #3643

It may be that this documentation change is considered a fix to that issue, so in the spirit of the contribution guidelines: Fixes #3643

Screenshots / GIFs

Tests

• With a complete configuration which doesn't include msc4143_enabled, attempt a call from Element X Android. This fails.
• Set msc4143_enabled to true and restart Synapse. Attempt a call from Element X Android. This succeeds.

Checklist

• I have read through CONTRIBUTING.md.
• Pull request includes screenshots or videos if containing UI changes
• Tests written for new code (and old code if feasible).
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[CLAassistant]

All committers have signed the CLA.

[her001]

It seems I was premature in this, as now when I have the transports endpoint properly set, Element X Android still works, but Element Web/Desktop fails. Disabling the feature has Element X Android still working, but makes Element Web/Desktop work (at least in more cases). I am marking this as a draft, as it may still be needed later.