thread_pthread.c (rb_sigwait_sleep): add note about spurious wakeup

I already forgot why we needed to jump through such hoops :x
[ruby-core:88102]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65648 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_pthread.c

@@ -1981,6 +1981,9 @@ rb_sigwait_sleep(rb_thread_t *th, int sigwait_fd, const rb_hrtime_t *rel)
          * tricky: this needs to return on spurious wakeup (no auto-retry).
          * But we also need to distinguish between periodic quantum
          * wakeups, so we care about the result of consume_communication_pipe
+         *
+         * We want to avoid spurious wakeup for Mutex#sleep compatibility
+         * [ruby-core:88102]
          */
         for (;;) {
             const rb_hrtime_t *sto = sigwait_timeout(th, sigwait_fd, &to, &n);