docs(nuttx): document px4_task_join limitations

NuttX tasks are task_create() children, not pthreads, so px4_task_join has no
portable join and instead polls whether the task still exists. Document that it
returns no exit status, can wait on the wrong task if the PID is reused, and is
only safe because its sole on-target caller is WorkQueueManager shutdown (and
NuttX shutdown is a full reboot).

Signed-off-by: Julian Oes <julian@oes.ch>

platforms/nuttx/src/px4/common/tasks.cpp

@@ -94,7 +94,12 @@ int px4_task_delete(int pid)
 
 int px4_task_join(int pid)
 {
-	// Wait for the task to exit by polling whether it still exists
+	// LIMITATION: NuttX tasks are task_create() children, not pthreads, so there is
+	// no portable join here. This waits by repeatedly checking whether the task is
+	// still alive. It returns no exit status, and can wait on the wrong task if the
+	// PID gets reused. It is only safe because the only on-target caller is
+	// WorkQueueManager shutdown, and NuttX shutdown is a full reboot, so this
+	// effectively never runs on hardware.
 	while (kill(pid, 0) == 0) {
 		px4_usleep(10000);
 	}