[backup] add code comment to UserKeys definition explaining Olm acc…

…ount might be out of date

Summary: [ENG-9705](https://linear.app/comm/issue/ENG-9705/add-code-comment-to-userkeys-definition-explaining-olm-account-might).

Test Plan: N/A

Reviewers: varun, bartek, ashoat

Reviewed By: varun, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13857

native/native_rust_library/src/backup.rs

@@ -469,6 +469,12 @@ struct CompactionDownloadResult {
   backup_log_data_key: String,
 }
 
+/// Stores the Olm account in `pickled_account`. However, Olm account
+/// information might be out of date. We have decided we don't need
+/// to update this when one-time keys (OTKs) or prekeys change.
+/// The reasoning behind this decision is that the backed-up Olm account
+/// is primarily used for signing an update to the device list. For these
+/// operations only the identity signing key is necessary.
 #[derive(Debug, Serialize, Deserialize)]
 struct UserKeys {
   backup_data_key: String,