pimd, pim6d: clarify RFC5549 MRIB nexthop handling

pimd, pim6d: clarify RFC5549 MRIB nexthop handling

The entire `case NEXTHOP_TYPE_IPV6_IFINDEX:` block here was a bit of a
tripwire to stumble over, since there was no indication at all that this
concerns RFC5549 nexthop handling.  So it got mis-adapted for PIM IPv6
support.

Clarify this a whole bunch that this is for v4-over-v6 nexthop mangling,
and nothing else.

This should really also use neighbor's secondary address lists for the
lookup, but that is probably going to break compatibility with other
boxes that don't include v6 addrs in v4 hellos and needs further
machinery to do properly, so for now just leave a breadcrumb.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>