DOC: fd: make it clear that some fields ordering must absolutely be respected

DOC: fd: make it clear that some fields ordering must absolutely be respected

fd_set_running() and fd_takeover() may both use a double-word CAS on the
(running_mask, thread_mask) couple and as such they expect the fields to
be exactly arranged like this. It's critical not to reorder them, so add
a comment to avoid such a potential mistake later.