Add encode_scan_progress documentation

Co-authored-by: Daira Emma Hopwood <daira@jacaranda.org>

backend-lib/src/main/rust/lib.rs

@@ -1147,6 +1147,8 @@ pub unsafe extern "C" fn Java_cash_z_ecc_android_sdk_internal_jni_RustBackend_ge
     unwrap_exc_or(&env, res, -1)
 }
 
+/// Returns a `JniScanProgress` object, provided that numerator is nonnegative, denominator
+/// is positive, and the represented ratio is in the range 0.0 to 1.0 inclusive.
 fn encode_scan_progress(env: &JNIEnv<'_>, progress: Ratio<u64>) -> Result<jobject, failure::Error> {
     let output = env.new_object(
         "cash/z/ecc/android/sdk/internal/model/JniScanProgress",