reformat-indexes info

a quick blurb should be added to the `riak-admin` and upgrading sections on "reformat-indexes" [1]. Reported by user on riak mailing list [2]

[1] https://github.com/basho/riak/blob/1.3/RELEASE-NOTES.md#2i-big-integer-encoding
[2] http://lists.basho.com/pipermail/riak-users_lists.basho.com/2013-May/012127.html