Specify fallback keys (#3615)
* Specify fallback keys
MSC: https://github.com/matrix-org/matrix-doc/pull/2732
* changelog
* Appease spell check
* Fine, let's appease the spellcheck this way
* Apply suggestions from code review
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>
* Fix intro
* word wrap
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>
