add #![deny(missing_docs)] AND #![deny(warnings)] in main.rs, and add more comments

os/src/syscall/process.rs

@@ -3,17 +3,20 @@
 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 current time 
 pub fn sys_get_time() -> isize {
     get_time_ms() as isize
 }