dragonball: Fix clippy missing_docs

Fix `missing_docs` clippy warning as suggested by rust 1.85.1.

```console
error: missing documentation for an associated function
    --> src/device_manager/mod.rs:1299:9
     |
1299 |         pub fn new_test_mgr() -> Self {
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `-D missing-docs` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(missing_docs)]`
```

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

src/dragonball/dbs_utils/src/rate_limiter.rs

@@ -556,7 +556,8 @@ mod tests {
             self.processed_refill_time
         }
 
-        // After a restore, we cannot be certain that the last_update field has the same value.
+        /// Helper function to help comparing `TokenBucket` after update.
+        /// After a restore, we cannot be certain that the last_update field has the same value.
         pub fn partial_eq(&self, other: &TokenBucket) -> bool {
             (other.capacity() == self.capacity())
                 && (other.one_time_burst() == self.one_time_burst())

src/dragonball/src/device_manager/mod.rs

@@ -1296,6 +1296,7 @@ mod tests {
     }
 
     impl DeviceManager {
+        /// Helper function to create a dummy DeviceManager for testing purpose
         pub fn new_test_mgr() -> Self {
             let kvm = Kvm::new().unwrap();
             let vm = kvm.create_vm().unwrap();

src/dragonball/src/vm/mod.rs

@@ -874,6 +874,8 @@ impl Vm {
     }
 }
 
+/// Unit test module for methods of struct Vm, and helper functions to shorten
+/// test code.
 #[cfg(test)]
 pub mod tests {
     #[cfg(target_arch = "aarch64")]
@@ -889,6 +891,7 @@ pub mod tests {
     use crate::test_utils::tests::create_vm_for_test;
 
     impl Vm {
+        /// Helper function to forch change `mstate` of Vms
         pub fn set_instance_state(&mut self, mstate: InstanceState) {
             self.shared_info
             .write()
@@ -897,6 +900,7 @@ pub mod tests {
         }
     }
 
+    /// Helper function to create dummy Vm instance for testing purpose
     pub fn create_vm_instance() -> Vm {
         let instance_info = Arc::new(RwLock::new(InstanceInfo::default()));
         let epoll_manager = EpollManager::default();