Clarify sibling creation.

Siblings can be created inside Riak regardless of the `allow_mult`
setting. The previous wording implies that `allow_mult = true` must be
set for this to happen.

Later on the document explains that only `last_write_wins = true` will prevent sibling creation inside of Riak.

Maybe we should use this ML message as a guide for reworking this?

http://lists.basho.com/pipermail/riak-users_lists.basho.com/2011-November/006325.html