Add an explanatory comment to the test.

Author: Vytautas Astrauskas

tests/compile-fail/concurrency/libc_pthread_join_multiple.rs

@@ -10,6 +10,7 @@ use std::thread;
 use std::{mem, ptr};
 
 extern "C" fn thread_start(_null: *mut libc::c_void) -> *mut libc::c_void {
+    // Yield the thread several times so that other threads can join it.
     thread::yield_now();
     thread::yield_now();
     ptr::null_mut()