Update docs

os/src/syscall/mod.rs

@@ -9,6 +9,7 @@ mod process;
 use fs::*;
 use process::*;
 
+/// handle syscall exception with `syscall_id` and other arguments
 pub fn syscall(syscall_id: usize, args: [usize; 3]) -> isize {
     match syscall_id {
         SYSCALL_WRITE => sys_write(args[0], args[1] as *const u8, args[2]),

os/src/syscall/process.rs

@@ -1,17 +1,20 @@
 use crate::task::{exit_current_and_run_next, suspend_current_and_run_next};
 use crate::timer::get_time_ms;
 
+/// task exits and submit an exit code
 pub fn sys_exit(exit_code: i32) -> ! {
     println!("[kernel] Application exited with code {}", exit_code);
     exit_current_and_run_next();
     panic!("Unreachable in sys_exit!");
 }
 
+/// current task gives up resources for other tasks
 pub fn sys_yield() -> isize {
     suspend_current_and_run_next();
     0
 }
 
+/// get time in milliseconds
 pub fn sys_get_time() -> isize {
     get_time_ms() as isize
 }