Add a comment warning against use of pg_usleep() for long sleeps.

Follow-up to commit 873ab97219, in which
I noted that WaitLatch was a better solution in the commit log message,
but neglected to add any documentation in the code.

src/port/pgsleep.c

@@ -29,6 +29,16 @@
  * the requested delay to be rounded up to the next resolution boundary.
  *
  * On machines where "long" is 32 bits, the maximum delay is ~2000 seconds.
+ *
+ * CAUTION: the behavior when a signal arrives during the sleep is platform
+ * dependent.  On most Unix-ish platforms, a signal does not terminate the
+ * sleep; but on some, it will (the Windows implementation also allows signals
+ * to terminate pg_usleep).  And there are platforms where not only does a
+ * signal not terminate the sleep, but it actually resets the timeout counter
+ * so that the sleep effectively starts over!  It is therefore rather hazardous
+ * to use this for long sleeps; a continuing stream of signal events could
+ * prevent the sleep from ever terminating.  Better practice for long sleeps
+ * is to use WaitLatch() with a timeout.
  */
 void
 pg_usleep(long microsec)