Add more context for why threaded vs unthreaded read receipts (#3378)
* Add more context for why threaded vs unthreaded read receipts
See https://github.com/matrix-org/matrix-js-sdk/pull/3339#discussion_r1195364120
* Language updates from Andy
See https://github.com/matrix-org/matrix-js-sdk/pull/3378#discussion_r1197622113
* Fix lints