Add a comment clarifying why we conclude Unknown for an historical me…

…ssage.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Signed-off-by: Andy Balaam <mail@artificialworlds.net>

crates/matrix-sdk-crypto/src/types/events/utd_cause.rs

@@ -220,6 +220,8 @@ impl UtdCause {
             if crypto_context_info.is_backup_configured
                 || crypto_context_info.this_device_is_verified
             {
+                // Either backup download is working but we still don't have the key, or backup download
+                // *isn't* working despite being verified. In either case, we shrug and give an `Unknown` cause.
                 UtdCause::Unknown
             } else {
                 UtdCause::HistoricalMessageAndDeviceIsUnverified