docs: add docstring for upload_file method

src/client.rs

@@ -261,6 +261,13 @@ impl Client {
             .map_err(crate::Error::SshError)
     }
 
+    /// Upload a file with sftp to the remote server.
+    ///
+    /// `src_file_path` is the path to the file on the local machine.
+    /// `dest_file_path` is the path to the file on the remote machine.
+    /// Some sshd_config does not enable sftp by default, so make sure it is enabled.
+    /// A config line like a `Subsystem sftp internal-sftp` or
+    /// `Subsystem sftp /usr/lib/openssh/sftp-server` is needed in the sshd_config in remote machine.
     pub async fn upload_file(
         &self,
         src_file_path: &str,