Followup to r362502: rc.conf(5): unobsolete gif_interfaces

There are cases when gif_interfaces cannot be replaced
with cloned_interfaces, such as tunnels with external IPv6 addresses
and internal IPv4 or vice versa. Such configuration requires
extra invocation of ifconfig(8) and supported with gif_interfaces only.

Fix manual page and provide some examples.

MFC after: 1 week
X-MFC-With: 362502