[libc][wait] add comment about __W_CONTINUED value (#125934)

Author: nickdesaulniers

libc/src/sys/wait/wait4Impl.h

@@ -65,6 +65,8 @@ LIBC_INLINE ErrorOr<pid_t> wait4impl(pid_t pid, int *wait_status, int options,
       *wait_status = W_STOPCODE(info.si_status);
       break;
     case CLD_CONTINUED:
+      // Set wait_status to a value that the caller can check via WIFCONTINUED.
+      // glibc has a non-POSIX macro definition __W_CONTINUED for this value.
       *wait_status = 0xffff;
       break;
     default: