review: better comments

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

@@ -48,8 +48,12 @@ pub enum UtdCause {
     /// obtained from a legacy (asymmetric) backup, unsafe key forward, etc.)
     UnknownDevice = 4,
 
-    /// We are missing the keys for this event, but it is an historical message
-    /// and no backup is accessible or usable.
+    /// We are missing the keys for this event, but it is a "device historical"
+    /// message and no backup is accessible or usable.
+    /// Device historical means that the message was sent before the current
+    /// device existed, but the current user is a member of the room. Not to
+    /// be confounded to pre-join or pre-invite messages (see
+    /// `SentBeforeWeJoined` for that).
     HistoricalMessage = 5,
 }
 
@@ -69,17 +73,17 @@ enum Membership {
     Join,
 }
 
-/// Contextual crypto information used to properly [`UtdCause::determine`] an
-/// Unable-To-Decrypt cause in addition to the [`UnableToDecryptInfo`] and raw
-/// event info.
+/// Contextual crypto information used by [`UtdCause::determine`] to properly
+/// identify an Unable-To-Decrypt cause in addition to the
+/// [`UnableToDecryptInfo`] and raw event info.
 #[derive(Debug, Clone, Copy)]
 pub struct CryptoContextInfo {
     /// The current device creation timestamp, used as a heuristic to determine
     /// if an event is device historical or not (sent before the current device
     /// existed)
     pub device_creation_ts: MilliSecondsSinceUnixEpoch,
     /// True if key storage is correctly set up and can be used by the current
-    /// client.
+    /// client to download and decrypt message keys.
     pub is_backup_configured: bool,
 }