rtsold: Note network interfaces changing index when removed/inserted

Remove reference to PCMCIA. The issue is more generic than that. Also,
it does apply to FreeBSD, so no need to hedge about some OSes. The index
won't change if no other interfaces are created after the card is
removed, so note that it may change, not will change.

Suggested by: phk
Reviewed by: gleb
Sponsored by: Netflix