Add note about MSC3886, MSC3903 and MSC3906 being closed

[hughns]

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[dbkr]

Thanks, this will be very helpful!

[richvdh]

@dbkr @hughns I'm assuming the only reason this hasn't already been merged is that you both forgot about it...