docs(sdk): Explain why it may be problematic to manually verify devices

crates/matrix-sdk/src/encryption/identities/devices.rs

@@ -216,6 +216,16 @@ impl Device {
     ///
     /// [`Client::cross_signing_status()`]: crate::Client::cross_signing_status
     ///
+    /// ### Problems of manual verification
+    ///
+    /// Manual verification may be more convenient to use, i.e. both devices need
+    /// to be online and available to interactively verify each other. Despite
+    /// the convenience, interactive verifications should be generally
+    /// preferred. Manually verifying a device won't notify the other device,
+    /// the one being verified, that they should also verify us. This means that
+    /// device `A` will consider device `B` to be verified, but not the other
+    /// way around.
+    ///
     /// # Examples
     ///
     /// ```no_run