Update redis.conf "bind" directive comment

Update the comment  to mention last-bind-wins
as well as add some IPv6 addresses to the
commented out examples.

Rationale
---------
I always thought "bind" was like "save" where
every later usage of the directive would be
aggregated together and processed as a bundle at
config bring-up time, but it turns out each read
of the "bind" directive erases prior usage.

Except, not completely.  If you have:

    bind 172.16.1.1 172.16.1.2 172.16.1.3
    bind 172.16.1.6

The second "bind" will make the server only bind to 172.16.1.6
(since server.binaddr_count gets set to 1 for that evaluation
of the "bind" directive and listenToPort() will only loop through
binaddr_count elements of server.bindaddr[]), *BUT* the other
positions in server.bindaddr[] will still be populated by the
IPs from the prior "bind" minus the first IP which got overridden
by the second "bind."

So, we end up wtih:

    bindaddr[0] = 172.16.1.6
    bindaddr[1] = 172.16.1.2
    bindaddr[2] = 172.16.1.3

They remain populated, but will never be used since: the maximum
evalation is server.bindaddr[server.bindaddr_count-1] *and* Redis
maintains no mechanism to alter, update, or re-bind IP addresses
at runtime, so extra addresses can't impact operations.

We're left with three possibilities for future improvements,
each is completely optional:

Optional fix A: after each evaluation of a "bind", run
sdsfree() all higher array indexes.

Optional fix B: make "bind" an additive directive where each
additional line adds new addresses (two line change, but may
break exising configurations relying on old last-line-wins
behavior).  This may also break some configurations where people
are overriding the config "bind" line with a command line --bind
(which gets evaluated last, and therefore overrides any mention
of "bind" currently in a redis.conf).

Optional fix C: Just leave it alone since there's no functional
impact.
Cluster: Add redis.conf comment about "bind"

Add note about the first bind address being used as
be the source address for contacting other cluster nodes,
so the first bind address must be reachable from all
other cluster nodes you expect to talk to.

We can give more examples and use cases of these sutations
in the official redis.io docs too.