Update doc

Author: chong-he

book/src/validator-manager-api.md

@@ -55,9 +55,9 @@ Validator 0x8de7ec501d574152f52a962bf588573df2fc3563fd0c6077651208ed20f24f3d8572
 
 You can safely shut down the validator client at this point.
 
-The following exit command will only generate the exit message and will not publish the message to the network, i.e., there will be no voluntary exit.
+The following command will only generate the voluntary exit message and save it to a file named `{validator_pubkey}.json`. It will not publish the message to the network, i.e., there will be no voluntary exit.
 
-To only generate the exit message without publishing it to the network, use the flag `--signature`:
+To generate the exit message and save it to a file, use the flag `--signature`:
 
 ```bash
 lighthouse vm exit --vc-token ~/.lighthouse/mainnet/validators/api-token.txt --validators all --signature

validator_manager/src/exit_validators.rs

@@ -7,6 +7,7 @@ use eth2::{BeaconNodeHttpClient, SensitiveUrl, Timeouts};
 use serde::{Deserialize, Serialize};
 use serde_json;
 use slot_clock::{SlotClock, SystemTimeSlotClock};
+use std::fs::write;
 use std::path::PathBuf;
 use std::time::Duration;
 use types::{ChainSpec, EthSpec, PublicKeyBytes};
@@ -209,7 +210,7 @@ async fn run<E: EthSpec>(config: ExitConfig) -> Result<(), String> {
                         exit_message_all.push(json.clone());
                     } else {
                         let file_path = format!("{}.json", validator_to_exit);
-                        std::fs::write(&file_path, json).map_err(|e| {
+                        write(&file_path, json).map_err(|e| {
                             format!("Failed to write voluntary exit message to file: {}", e)
                         })?;
                         println!("Voluntary exit message saved to {}", file_path);
@@ -361,7 +362,7 @@ async fn run<E: EthSpec>(config: ExitConfig) -> Result<(), String> {
     if merge {
         let all_json = serde_json::to_string(&exit_message_all)
             .map_err(|e| format!("Failed to serialize voluntary exit message: {}", e))?;
-        std::fs::write("all_validators.json", all_json)
+        write("all_validators.json", all_json)
             .map_err(|e| format!("Failed to write all voluntary exit messages to file: {}", e))?;
         println!("All voluntary exit messages save to all_validators.json.")
     }