LibC: Add POSIX spec links to `wait`, `waitpid` and `waitid`

Userland/Libraries/LibC/sys/wait.cpp

@@ -12,11 +12,13 @@
 
 extern "C" {
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html
 pid_t wait(int* wstatus)
 {
     return waitpid(-1, wstatus, 0);
 }
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/waitpid.html
 pid_t waitpid(pid_t waitee, int* wstatus, int options)
 {
     siginfo_t siginfo;
@@ -73,6 +75,7 @@ pid_t waitpid(pid_t waitee, int* wstatus, int options)
     return siginfo.si_pid;
 }
 
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/waitid.html
 int waitid(idtype_t idtype, id_t id, siginfo_t* infop, int options)
 {
     Syscall::SC_waitid_params params { idtype, id, infop, options };