Mention portMAX_DELAY in xEventGroupWaitBits docs (#411)

The public function xEventGroupWaitBits passes xTicksToWait to the
function vTaskPlaceOnUnorderedEventList, which passes the number of
ticks to prvAddCurrentTaskToDelayedList and sets xCanBlockIndefinitely
to pdTRUE, causing the latter to block indefinitely if
xTicksToWait == portMAX_DELAY and INCLUDE_vTaskSuspend == 1.
diff --git a/include/event_groups.h b/include/event_groups.h
index dd2fda8..6807002 100644
--- a/include/event_groups.h
+++ b/include/event_groups.h
@@ -243,7 +243,8 @@
  *
  * @param xTicksToWait The maximum amount of time (specified in 'ticks') to wait
  * for one/all (depending on the xWaitForAllBits value) of the bits specified by
- * uxBitsToWaitFor to become set.
+ * uxBitsToWaitFor to become set. A value of portMAX_DELAY can be used to block
+ * indefinitely (provided INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).
  *
  * @return The value of the event group at the time either the bits being waited
  * for became set, or the block time expired.  Test the return value to know