Add note about MSC3886, MSC3903 and MSC3906 being closed

[hughns]

Type of change

• [ ] Feature
• [ ] Bugfix
• [ ] Technical
• [x] Other : Documentation/Comments

Content

Motivation and context

Screenshots / GIFs

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• [ ] Physical
• [ ] Emulator
• OS version(s):

Checklist

• [ ] Changes has been tested on an Android device or Android emulator with API 21
• [ ] UI change has been tested on both light and dark themes
• [ ] Accessibility has been taken into account. See https://github.com/element-hq/element-android/blob/develop/CONTRIBUTING.md#accessibility
• [ ] Pull request is based on the develop branch
• [ ] Pull request includes a new file under ./changelog.d. See https://github.com/element-hq/element-android/blob/develop/CONTRIBUTING.md#changelog
• [ ] Pull request includes screenshots or videos if containing UI changes
• [ ] Pull request includes a sign off
• [ ] You've made a self review of your PR
• [ ] If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()